Why is it okay to winch in the mainsheet at your leisure before the gybe, if it's so important to let it out quickly on the other? Do you not risk a pre-gybe broach if you spend too much time with the main close to center prior to the turn?
I think this has been well answered by SD and nolatom, but I'll add that a gybe cannot occur unless the wind becomes parallel to the length of the boom. It's a frequent occurrence that new sailors (particularly those who have taken the beginner's courses) are often scared so silly by their instructors about accidentally jibing when on a run that they forget (or don't realize or aren't taught) that if the boom is all the way out (say 70 degrees), you still have to turn 70 more degrees before a jibe can occur.
When you're sheeting in, as long as you're not by the lee, not in steep following seas or on a dead run, sheeting in is a very safe part of the maneuver.