Hi, Steve,
You can set a default for the field for new customers to use when created. However, there's no template for customer entry that would insist that the field be visited and dealt with.
Tax Group would have to be a required field and be blank. I can enter a suggestion to R&D for a future release if you'd like.
Of course, there may be other sites that really don't like that behavior though.
_________________________
Regards,
Softrak Tech Support