I have Sunguard window tint, that was professionally installed over my defroster, been there since January 2000. I use my rear defroster regularly and have encountered no problems. Just ask your installer and verify this before having it installed.
They have always put the tint over the grids and only cut the tint on the back window and hide the cut using the grids to allow the tint to convex for the glass shape.
If you've had tint installed by the dealership, like I have, and it starts bubbling, can I get it replaced with no questions asked? It really looks like crap, and I hate the way it looks right now.
removing it off the back window is also no problem as long as the film isn't REALLY old. If it gets really really old and won't peel off...you have to scrape it which will render your defroster lines useless.
and yes, your dealership will probably replace your tint as long as there was somekind of warranty with it (most give lifetime warranties now).