Ask HN: Is it okay to ask friends to upvote a submission?
Something like, "if you like this, please upvote it on Hacker News." Is that, you know, kosher to tweet n stuff?
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.