Yes, we have an equine vet who specialises in acupuncture and it can be very helpful for many horses.
I would always want to first get to the bottom of why the muscles are tight first though as it's often an indication of a problem.
I have had it done to a horse years ago, and the results were quite dramatic. It made me believe that accupunture works and is not reliant on the patient thinking it was going to work - if that makes sense.
It never cured him, and he was eventually PTS, but it meant that he was 'pain' free for a few months and rideable.
Her muscles are tight as she uses her neck and shoulders to block against my hands, she is a very flighty mare with a dodgy past!! She has regular mctimoney treatments so was just looking for anything else which may help. I know it helps me with my back!