Boxing the wake is essential to learning how properly control a glider on tow. I box the wake on virtually every tow, just to keep my skills sharp. If your tow plane can't handle this stress, it needs some structural reinforcement.

Boxing the wake might teach essential skills but if it's doing damage to a tow plane, why on earth should we continue the practice. Develop other training methods to teach that skill set. It's similar to the requirement in the Powered Aircraft training which requires a commercial pilot applicant to perform a "Lazy Eight". I'm a CFI and I can tell you that a lot of money is wasted teaching that maneuver for the commercial check ride. I'd bet that darn few of those commercial students can perform that maneuver 6 months after the check-ride. It's nothing more than a CFI meal ticket.