It depends. In leveling dungeons, you should do trash pulls with your tank stance on, generally. Because leveling dungeons have far stricter syncs than the level cap dungeons (i.e., level 50 dungeons, level 60 dungeons, and level 70 dungeons), and the trash mobs absolutely truck in there. In level cap dungeons, I have seen a few tanks pull trash without their stance on, maintain aggro, and not be a royal PITA for me to heal—but these are also tanks that play at a high level, so they know how to manage their cooldowns and their aggro well. This will not work for inexperienced tanks.
My advice would be to stay in tank stance for most trash pulls, and drop it for bosses in most content—you will first want to generate enough aggro on the boss before you drop stance, so that it won’t turn around and smack a DPS. However, keep in mind that if you do lose hate, it may not be entirely on you. You can pop into stance if you feel like aggro may be slipping away from you.