I'm using a Tamron 17-50 VCII lens on a 7D and 2Ti. I can turn the focus ring while AF is engaged fairly easily to focus (not while camera is trying to autofocus!) . Is this unduly hard on the AF drive as long as the motor isn't actively trying to focus?

It would be nice to AF at the start of a shot and avoid the shakes and noise of turning off AF during the same shot. Don't want to damage the lens, however.

I have the same lens and a Canon Ef 50mm 1.4
The Canon lens definitely can be manual focused while AF is on, but the Tamron feels really weird when pulling focus while AF is on. I wouldn´t do it.

Besides the speed of the lens, how do images from the Tamron hold up to the 50mm f1.4? (yeah, I know it's not a fair comparison)

The Tamron does not have a USM motor, so manual-focus while AF is engaged is not suggested - the gears are only plastic, and you may eventually grind them out. Tamron is rolling out a USM style mechanism, but it's not available yet on most of their lenses.

Many of the Canon's have USM and allow for FTM (Full-Time Manual - look for this keyword), so you can always manually focus the lens even while AF is engaged.

There's advantages and disadvantages either way. USM style lenses do not have hard stops, you can keep spinning the focus wheel forever.

Thanks guys, I'll refrain from manual focusing while in AF unless it keeps me from missing a crucial shot.