Throwing in some bicep work every workout... overtraining?

I currently have my split as:

Day 1 - Chest/Bi's
Day 2 - Legs
Day 3 - Back/Tri's
Day 4 - Cardio
Day 5 - Shoulders/Traps
Day 6 - Cardio
Day 7 - Off (or maybe cardio such as pickup basketball)

Now I hit my biceps pretty hard on bicep day, and they get some work in on back day, but I'm wondering if it's too much to throw in one set on days that I lift.

Example:
On back/tri's day, I throw in 3 sets of dumbbell hammer curls.
On shoulders/traps, I throw in 3-4 sets of preacher curls.

Those two exercises, hammer and preacher curls, I don't do on my bicep days.

And on top of that, would doing 2 sets of very light (15 lbs) reverse curls on cardio and leg days be bad?

So Day 1 - normal biceps
2 - reverse curls (high rep, very light weight, 2 sets)
3 - back day plus hammer curls
4 - reverse curls
5 - preacher curls
6 - reverse curls
7 - off

My biceps obviously need work, but I don't want to over train them.

Make two bicep days if you want to focus on them, but I wouldn't sprinkle bicep workouts throughout your week. I'd also keep your total working sets at a reasonable level. If you're bringing the right intensity to a bicep workout you don't need to do five dumb exercises.

Yes definitely that is overtraining. I train them once a week directly then once a week indirectly through my back workout, and my biceps are BIG.

Hey, don't worry, everyone gets the idea that "more is better" applies to everything -- and sometimes it works, but not in bodybuilding.

Just hit them real hard on bicep day; and on back day, invert your grip on low cable rows and on lat pulldowns. That will really activate your biceps. If you want, at the end of your back workout, you might do some barbell curls, but no more than 3 sets.

That is more than enough to get 'em big.