don't sweat it. like everyone else said, so long as you don't exceed the minimum mark, it should be ok. a decent (read: not stupid light) post should be fine. it can be more of an issue on frames with slacker seattubes, just by the nature of the stresses placed on them vs. a steeper angle.

i've got a lot of post showing on both my bikes. no problems. and on my roadie as well.