You will want to make it easy for your artist to get to the area of the tattoo, but never do anything that makes you uncomfortable. If your artist makes you feel uncomfortable, walk out. That being said, I think the best thing for you to wear in this case would be a skirt or dress. Not tight, and not fancy though. You will need to be able to pull it up enough, and you don't want to get ink on a nice dress. If you can't stand the thought of wearing a skirt or dress, I would go with a pair of shorts that are really loose in the leg and can be pulled up enough.
Pulling your trousers down or taking them off would be the last option if you are concerned about showing too much. I think the other 2 ways make more sense. You can also bring along a towel or blanket to cover up your lower legs if you're a little shy - just tell them you're cold.