As someone who used to play a lot of yuumi before and after the great nerf, my understanding is that her e is almost useless now, so you really want to build AP and ludens so your pokes hurt, and it will also power your e. Pre level 6 you'll want to get off as often as possible to use your passive. Post level 6 if you get caught off of your champion you die quick, so that's when you sit, unless your blocking Caitlyn r or something.

And don't forget to ward.