While it's good that you want to do things "properly", the DPS can see which mob you're attacking from the one which your axe/shield goes flying at and starts losing HP first, and if you're manually marking targets on every single pull they're going to get bored and impatient.

You really don't need to mark targets in dungeons where there's only 2-3 mobs per pull and no particularly compelling reason to kill them in a specific order, but if you must, set up an actionbar with all your marks on it and bind it to your numpad keys or something. Most mobs won't really do much damage even to non-tanks until fairly late on anyway, so while it can be annoying trying to tank for a bunch of hyperactive synced-down DPS, it's really not worth getting stressed out over.