Which is why, to be honest, I still favour suits - their universality. Other dress codes require a far greater cultural insight to properly judge, both regionally and of the client culture, yet in almost any practical context we're likely to find ourselves within IT, a suit works as a starting point. If your suit isn't comfortable and don't fit then you've found a bad suit; find a better one, which can still be from an ordinary high street retailer and be cheaper than you'd expect.
If it were me searching my wardrobe... Hmm, the pinstriped jacket is clearly out ;-) Black designer suit - no, too harsh/loud and I'd look like the cleaners had mangled the trousers so I had to grab jeans as the only available alternative. Grey slim cut suit - again, too harsh, plus I can't visualise the tonal combination working with the softish blue jeans (think loose fitting casual, not 501s) and plain white t-shirt I've also picked out, or another clothing scheme it'd work with. Softer black jacket (I can't think how to describe the fabric but it's on the moleskin-felt spectrum), OK, that'll work - it's plain enough that it doesn't look out of place with the rest, it's smart enough that it's making an effort to lift it. But, if it were me, I'd be wearing a nice plain casual shirt instead of a t-shirt.
- the sleeve length should be to your hand when your hand is bent back 90 degrees. It can be slightly shorter for a casual jacket.
- there should not be any bunching or lifting on the area under the collar at the back when you stand upright normally. This one is very common.
- it should not be too tight anywhere when buttoned. You should be able to clench your shoulders and arms with your arms out in front without it pulling under your armpits or pulling too much across the shoulders.
Investors are a totally different beast. In my opinion, most of them aren't going to be too concerned about how you dress other than judging if you present yourself well to your customers/clients (see above).
On the other hand, I've known others (though surprisingly few) who couldn't look less comfortable and at home in a suit if you cut anatomically unfortunate holes in it and poured in itching powder. For them it's clearly a net loss, but I think many people unnecessarily put themselves in that group.
After watching that, I developed the following opinion:
Any time somebody is evaluating you (for investment, for a job etc) they are going to have an idea in their head of what they want. They will put you through a filtering process and decide whether or not to let you through.
What most people don't consider is that the filter is not just there to help the decison maker, it's there to help them too. You get to present yourself however you want so you can choose the inputs to the filter.
If you don't want to have an investor who would think your choice of clothing is important, don't wear a suit. If you don't want to work somewhere that chooses to interview people based on the amount of bullshit in their CV, don't put bullshit in yours, and so on.
edit: I forgot to address the question with regards to customers. In that case the answer seems to be "whatever the customer expects" since you presumably don't want to put a barrier in the way of them buying your stuff.
I've engaged and closed over $200k worth of investments for various projects I've worked on over the years, and it's never been an issue what attire I've worn to a meeting. I've even had several investors told me they liked that I wore a T-shirt and didn't wear a "monkey suit" to the meeting. "Leave it to the people that have to," they told me. I suppose I can agree with that. :)
If it makes you feel uncomfortable or gives you a 'dressed up monkey' feeling then better dress for your own comfort.
If you are used to wearing suits, look good in one and feel more confident then by all means wear a suit.
Pitching is all about confidence, anything that helps your confidence is good.
Being strong technically and having a good story as to how you have delivered value to previous customers and how you plan to deliver value to us is infinitely more important than how you are dressed.
[Edit: the best part of presentations is always to watch the face of the technical person when the sales person is giving their presentation - in my experience "real" technical folks don't have a poker face.]
The two are not mutually exclusive.
From what I've seen, a typical 'assault team' consists of: 'account manager' (usually clueless), 'sales manager' (same) and a 'technical person' (varies). There's hardly anything interesting coming out of the former two.
The 'techie' guy competence seems to be a function of a company size. The smaller the company, the better his/her pitch would be and more useful information you'd get out of him. If the company is large, they tend to send 'technical sales' chap, who usually knows a bit or two, but largely delivers 'I'll have to check this with our technical team and come back to you on this'.
Bear in mind, this is my largely technical POV. YMMV.
So, it depends on your audience. There's no one answer.
i once met a vc in a bar, when he came up to me and asked how i had managed to monopolize all of the beautiful women in the bar. at the time, i believe i was wearing eyeliner and clearly intoxicated.
i met the guy a week or two later, wearing torn jeans, a leather jacket, several rather elaborate rings, and a t-shirt that said 'proud to be awesome,' because that's the kind of clothes i normally wore at the time, and i didn't really think about what he'd think of it. we talked for a while and he gave me a hookup for a cto gig, i think largely because i made it evident how self confident i am.
I find it's more important to be an honest representation of yourself rather than be wrapped in a layer of turd polish. Also if you are judged on your appearance over your skills and/or product, then you really don't want to work with the other party.