I don’t know if this counts as a MIDI tip, but when I compose out of the studio (at the piano, or even at the local coffee shop, with pencil and paper) and the piece is finished and I start to build the MIDI mock-up sometimes I’ll record an audio track of me singing freely the main melody, with all the tempo fluctuations it dictates. Then I’ll make a tempo track using this guide, and carry on from there. One of the things I find most revealing of sample vs. real instruments is unnatural tight tempos. Of course this varies a lot with the genre, orchestration, etc., but on more lyrical pieces I found this to work for me very well.