I'm a Warrior and I have a macro that puts a "1" over the head of whoever I use tomahawk on. I usually then use overpower, followed by flash, and begin my single target rotation.

However, if the healer uses a bunch of high powered heals or if the dps attacks one of the adds I don't have marked, it is very possible to lose aggro. They I have to flash or tomahawk to try and grab aggro quickly again.

Overall, it's not too hard to keep threat. Even at level 35. The only time I run into any issue is with multiple targets. And even that is rare if everyone knows what they're doing.