Tipping at a SPA is that grey area between the service and an SB/SD relationship IMO. It depends what you're after. A spa trip can be as simple as enjoying your time, paying what's due and that's it, or you can do what many seem to do and get involved in having a few favourites, keeping up with their social media accounts where many will post gift ideas and every woman's favourite, endless messaging between bookings. For the extreme of this look up the twitter for Bella of Muse Toronto Spa.
If you are blown away by someone than go ahead and give an extra 20 and yes she will appreciate it but no more than they would at a restaurant.
If you're looking for a GFE experience that goes beyond the time booked, or is enhanced in the session, gifts are probably better than a bit of extra cash. Especially gift cards to common favourite shops like VS and Lulus. You can get ideas from their twitter accounts.
I definitely would not say tipping is expected though. Treat it as part of the fantasy if you're after that or if you do develop a favourite than surprise her with something around a big day. Never feel obligated to give more, or feel like you've set a precedent by doing it in the past.