I don't know if I would be comfortable with this kind of stuff. Hacker News is read by many people and if your post inherently has something to be liked, it will most likely be. You can then expect it to go viral from there.
But requesting that your friends upvote your posts shows that you're expecting to garner attention by the number of upvotes you've gathered. It doesn't matter if you say "upvote this if you like this", most friends will upvote just for your sake, some might not even take a look.
IMHO, just let the content of your posts/links speak for themselves.