Tom,
I generally only use PayPal for Ebay, so payment is instant.
Most often, if a seller is giving me feedback, they will do so when they are paid (which, again, is instantly).
Sometimes, a seller will send me a message saying "Thanks for your payment! I have left you positive feedback and hope you will do so when you receive the product." And usually, that's all the reminder I need to do so.
What I DON'T like is a seller who will keep positive feedback hostage until I give them feedback first. If I paid you in a timely manner, then I deserve my feedback.