people want to see them working hence have them on, old school, but now things have changed and the regs say they should be off but I cant remember the exact phrase used.
Wireless having off except walk test saves battery.
I have them on in my house (as it's reassuring they're working) but have the LEDs in the bedroom PIRs switch off when part set so they dont light up if I move around in bed in the dark.
.............. also, just think about it, LED On, it would be possible in some circumstances to do a "Walk Test" to judge the area of cover given prior to a break-in.