Urban legend. It's better to turn device off, cause ~8 hours of running idle will draw *much* more power than rebooting.

Exception to this is using of special power saving standby mode (phone wakes up from it in ~2 second), started from console. But, to use it, You must turn off-line mode on (in other case, phone will awake itself on every network probing). Of course, even with off-line mode, You need to manually put phone into standby. Anyway, if You're planing to not use it for few days, it's always better to turn it off.

Ps.

In real standby mode, Phone use ~40 mA in 8-9 hours.