An interesting idea really! Something worth considering for sure.Other than for reboots, I turned my phone off for 24 hours in the middle of December, a smart phone fast as part of a book I was reading on cell phone addiction. The book cited studies saying that doing this can have a positive effect on the way we use our phones for upwards of two weeks, I think it said? I really should put the phone away again.
To answer the initial question though, I only turn my iPhone off if there is a glitch/issues that needs to be fixed. Very very seldom though. It just seems easier to leave on all the time, and I like being reachable for some reason..