Mark or don’t mark required fields if all are required?

If all fields in a form are required should they be marked somehow (eg. with an asterisk)? I see this done a lot and find it redundant?

There could be a difference in big forms and small forms, but in big ones I would add a note at the top that all fields are required, but I wouldn't necessarily do that on small ones.

What do you think is the right approach?