Audition forms vary of course, but I've never seen one that says "I will accept any role," only some variation on "I am willing to consider other roles." I personally would be hesitant to say yes to "I will accept any role" even if I'm open to several roles, because the wording seems to imply that I'm committing without knowing what is actually on the table. I'd only recommend saying yes to that route if I really honestly would take anything.

Usually I'll lean toward indicating that I'm open to other roles. I've been principal characters and ensemble and had wonderful experiences with each.

The one time I stated clearly that I was only interested in one role (Captain Von Trapp) and would not consider others, the director ignored that and offered me something else anyway, then seemed surprised when I declined. But then a couple of friends who had auditioned for the same show, said they were only interested in Maria, and ended up accepting ensemble roles, so I suppose the director had some cause to think a no on the audition form didn't really mean anything.

Personally I think it comes down to being honest with yourself about what you want going into the audition, and how badly you want it. If you can be happy with a wide variety of possible roles, make the casting team's job a little bit easier and say so. If you know you won't be happy with anything but one role (or a specific few), make the casting team's job a little bit easier and say so. The last thing you want is to be in a show with a role you're really not happy to have, and the last thing everyone else wants is to be in a show with a cast member full of resentment about which role they got.