i have BX8s at home too, and they usually remain off when not in use. But they're on the same power conditioner as the rest of my gear. So I just have one easy switch for everything.
i have some passive Tannoy's at work which always remain on. nothing seems to have gone wrong with them (knock on wood)
I got bx5's. I always shut eveything off, including the power strip their all on, at night - unless for some reason I need to keep the comp running over night.
I would turn them off. My Behri truth's get pretty warm, which tells me that they're eating a good bit of current even idleing. Things that heat up generally have a finite life, so better to turn them off when not using them IMO.
Some monitors, such as my Mackie's, have an auto-on, auto-off feature where they shut off when they haven't received a signal for a given period of time (a few minutes). But I'm still obsessive about making sure everything in my studio is powered down when I'm finished using it for a while.