As an SP I always appreciate a tip, but never expect one. It's always nice to get rewarded for a job well done. Every client deserves to expect great service for the base rate, I realize and enjoy that it is my job and goal to go above and beyond to earn a tip or better yet, a repeat client.
Always tip after the session. You need to be able to gauge the service first and base your tip on that. Don't just tip because you feel like you have to or you feel it's expected. Tips should be earned and appreciated. When a client takes the time at the end of the session to tip it makes me feel fantastic because I know I made the client feel the same way.