I am sure from the point of view of the math behind steem, it is more profitable to buy upvote or self vote asap. In this case the curation rewards you will give are minimal. BUT here comes psychology and as you say motivation. IF your voters knew you will buy the big upvotes later then they upvote, they would probably upvote your post with bigger enthusiasm. Didn't test this theory though. I don't think it could work on my number of followers.