Curation rewards are based on how much upvoting a post receives after you upvote it. So if you get in before a lot of stake up votes, you get a better reward. If you're the last one to upvote, you get no reward. Everyone else gets at least a bit, theoretically.

After a post pays out, upvoting is pretty much just for aesthetic reasons, to say "hey, I still appreciated this." it doesn't pay more.

Note that Steem hard forks and changes the rules all the time, though, so my answers may be outdated already, or may become so in the future. :-P