I'm usually on a tank and I have to say that by and large, I keep aggro really well.

I don't usually lose hate in lvl50 content, when I do lose hate it is really one of two things.
1. I derp- I forget to take of regen, I miss a mob, etc <--- I do my best to man up and apologize to the group when this occurs.
2. DPS go balls to the walls before I've had a chance of establish hate <--- this really irritates me when they keep doing it. Thankfully it isn't common.

During low level dungeons, it's a lot easier to lose hate so I do mark more frequently in those.

That being said, I fully expect the whole party to work together and communicate regardless of which dungeon/instance is being run. If that means attacking mobs in the correct order then so mote it be. Taking the time at the start to establish the flow of the run is more beneficial than just assuming. ^^;