I leave mine on all the time, but I also charge my 8130 every night. At the end of the day, I notice my battery charge is at 85%.
Consequently, it seems to me that if you're faithful about charging your BB frequently, BT battery usage won't be a problem. I suppose I could switch BT off at the end of my working day and see if it makes a difference...perhaps I'll do that this weekend and see what happens.
There's an article about BB power conservation here...Five Tips: Get More Battery Life from Your RIM BlackBerry - CIO.com - Business Technology Leadership
it suggests switching BT off but says nothing on exactly how much power you save with any particular strategy...