Note that within the first 30 minutes, your curation rewards are split with the author. If you upvote something the second it is posted, 100% of your curation rewards go to the author. At 30 minutes out, 100% of the curation reward goes to you. In between, it's a linear function (so at 15 minutes, it's a 50/50 split).

So when you upvote something is somewhat of a strategic decision -- you want to get in early, but not TOO early, otherwise the author will get most of your curation rewards.