Upvoting own posts does not give you curation rewards. It does pay author rewards as usual, but no curation. In this sense it does not matter when you upvote. I'd say it's better to upvote early to increase visibility and rating of your post as soon as possible.
Curation is for upvoting others. This is the way Steem pays for finding and sharing good content.
RE: Estimate curation reward before voting