you can play with low enthusiasm. i did it for a long time because I didnt know how important enthusiasm is.
Having a higher enthusiasm does make your players play better, how much better i don't think noone knows exactly.
And with regards to your question when to play crunch time, again it all depends on the situation, opponent etc. Doing it during a tv game isnt a very bad idea, when I picked it up properly on the forums, doing well in a tv game raises the amount off people coming to your home games. So crunch time will make you play better so doing it for a tv game is a good option...