During the Def's bye week, some people will just drop their Def if there's one of equal value on the WW. If you have a particularly good Def that you don't want to lose, then drop someone else from your team (for example a WR that's not helping your team), then bench your regular Def and start the one you just picked up. After your bye, drop the extra Def you picked up and pick up a positional player. Alot of people only carry 1 TE
and 1 Kicker
as well and do the same thing with these positions during bye weeks.
And yes, normally you always start your "studs" unless they are hurt or on a bye. But sometimes you will have guys on your bench that may be performing as well as your starters, so then you play matchups (start the player who going against a weak opponent, and bench the guy going against a tough team). Also sometimes one of your stars may be on a real cold streak, or one of your bench players may be on a real hot streak, so that will help decide who you want to go with - but keep a close eye on them and be ready to take action if their streak ends.
Hopefully that's of some help and good luck.