I learned that the hard way. Offer to fill and so many times I would see someone pick a main tank. I say Ok cool ill heal… and then I watch the Rein (by choice) charge into the enemy over and over and over…
I went from struggling to stay in gold to mid plat just by saying screw it I play tanks. Only time I switch off is when there is other people that actually want to tank.
Fighting the urge to be helpful has been far better than trying to make a decent comp.