From my perspective as a SP, tips are never expected but always appreciated. My donation structure is set so that I always feel fairly compensated for the time and energy I give in dates.
A tip won't lead to me offering services I wouldn't otherwise, but think of it this way - it's one way among many to show appreciation for an SP you enjoy spending time with. (Other ways to show this: always being punctual and courteous, asking about their hobbies and interests in conversations, being easy-going and thoughtful). With clients who show these behaviours, I am likely to be more relaxed and comfortable, and this comes across in how well the two of us enjoy ourselves (or three, or more - couples and duos are fun too π )
Just my two cents!