[Sa'ar Chasm]

You see, going directly into warp from planetary orbit (or even within what I'd call inner stellar systems) creates a great deal of messy gravitational and subspace effects.

I seem to recall getting that impression from somewhere (ie, the Enterprise chasing the Borg ship in BOBW, dropping out of warp somewhere around Saturn and spending the next half hour crawling towards Earth), but it wasn't treated consistently (blasting into warp from orbit in the Schizoid Man, Kirk's BOP kicking into warp in the *atmosphere*, etc.)