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.
ok... thank you for the information and for creating awareness. I understand about upvoting for other posts :)
Actually you can get self curation by upvoting after 15 minutes, it works out to the same as if you voted when posting. The difference is it can add to the curation rewards of your followers and supporters.