As a healer who DPS's and pushes hard in dungeons, I still think that running ahead and actually pulling a trash pack is pretty idiotic. It's risking tension and drama within the group, you're likely going to get a cast or two interrupted if the tank is napping and as such, it's a net loss for everyone.

I usually get the best results being nice and civil about things if the tank is single pulling. 'Hey, I'm good for bigger pulls if you're up for it? The damage seems pretty decent'.

Where I push the line is doing things such as dotting mid pull and trying to pre cast on bosses. With most tanks it's no problem and I can time the cast reliably as long as they move with some degree of consistency.