Without knowing your specific situation, I think it's also problematic for two non-tax/appearances reasons:

1. Volunteers should be there to support the project, etc, not for personal profit. While there usually are benefits to the volunteer like learning new skills, if they're there for the money they're not really a volunteer. Setting out a tip jar sends the signal that the volunteer doesn't really understand their role there.

2. Policy changes at a co-op/collective should be a group decision imo. It's not much of a collective if everybody sets their own rules for the space. Maybe I misunderstood something but taking tips without clearing it with the rest of the collective kinda implies that the volunteer doesn't take the group process seriously.