While the Dining and Hotel staff are compensated with your fare, the Spa staff still relies on your tips for a part of their sustinence. Yes, it's most appropriate to tip them, just as if you were in a land based spa.
I had a wonderful treatment on the PG last year -A sunflower seed scrub which is followed by a shower and then a full massage. Hope they have it on the Voyager, where I am headed next.
