you can ask them for review but do not push them, for example you can add a line like this when you deliver your order.
“If you feel like I have delivered more then your expectations then please take your time to review me, as it will increase my worth and portfolio on fiverr”
I don’t dare to ask fro tips 
They’ll think I am crazy. After paying high, If I expect them to give a tip, I am being greedy. But yes, I ask for review when i think that the buyer loves my work and is really a nice person 
. If I feel that the client is a rude guy with no previous experience with a seller on fiverr and expected something magical, I wouldn’t bother asking for a review. Those are guys who leave amazing review
Yes, I appreciate you. 