is there a time of day/sun position that is preferable?
from a portrait standpoint (on the ground, and probably similar to in the air) late afternoon is the best time for that sort of thing.
If/when the sun is high pictures tend to come out flat but like others have implied the position of you relative to the sun relative to the subject is very important.
Its been said before (and it'll be said agian no doubt) get a good book about photography (or invest in a class). The concepts in a good book can be applied the skydiving photography... and frankly its easier to learn it on the ground first...