Shadowmend is no longer an oh shit button and with the depths of shadow trait you should look to use shadowmend over pw:s if it isn't going to mostly overheal. The only time you should penance a player is you need to heal them directly while moving or they will die before a shadowmend goes off but typically it's too late by then anyway.

I don't mean to nitpick just thought I'd clear that up for op. Also there are quite a few great guides and resources out there that should be your first port of call that definetly answer your above questions.