In FP's when I'm on my shadow main I try tank everything and I mostly succeed. Sometimes it requires very careful positioning and use of force pull, telekinetic wave and other tools. In ops we usually have our targets divided with co-tank and nothing gets left untanked. Apart from rare exception I do not really leave mobs for DPS to kill untanked unless specifically agreed or planned. Sometimes guardian DPS we have in a group will take one mob and deal with it, or even taunt it but that's rare exception (I remember one time in Op IX HM when we resorted to that for some reason, not sure why now).
When I'm on my guardian I feel more limited in options to hit everything and sometimes I will leave some weaks or normals for DPS to kill first. Most good healers can deal with 1 or two as well when they get to them damaged and barely alive. Actually one of the biggest concerns with healer players I have is when they keep shouting on DPS "I have an add on me" and I see them healing themselves from Titan-6 add that is like on 1% health instead of killing it with one attack
Anyway my point is - as a matter of training it's worth to try and tank everything and not use taunts unless extremely necessary. When I levelled my second tank in FP's I tried to tank everything even without AOE's and AOE taunt and mostly I could do it in SM levelling FP's. It's very taxing but possible. It will only help you later in operations