What do you think of putting less than +30? I've only ever done any large work with long standing clients that pay quickly. Would the majority of clients not agree to it? Could be a way to ensure that it ends up being +30 even with the client dragging their feet.
You can ask for whatever you want. You'll only run into problems with BigCo's if you do one of two bad things:
* Refuse to sign unless the BigCo agrees to your payment terms.
* Actually expect the BigCo to perceive time elapsing in a way that will get you paid in 30 +/- small n days.
They won't get offended when you ask for net+15. They might even agree. But you're getting paid when the stars align, not when you think the contract says you're getting paid. Established consultancies devote a small but palpable bit of effort managing this issue.
I did business with a department which was in the process of spinning off from a big Fortune 100 company. They had plenty of funds, but there was a lot of bureaucracy during the transition. As such, they told me they 'required' net-60
I asked for clarification, and they were apologetic about it. They could only run payments on a 60 day schedule. Solution: I billed net-30, 15% monthly surcharge for late payments, which they happily paid.
The lesson was that sometimes a client has no choice but to drag their feet, so be sure to give them that option (for a fee).