It's better to err on the side of marking. Most groups either want them or don't care, as long as it doesn't take you too long to do it. As you get more experience, you'll get a feel for when you need to mark and when you don't. I generally start out marking, then drop it if it seems like the group can handle things without it. Then pick it up again if it turns out I was wrong.

You can also just ask your groups if they want you to mark or not.