I sleep my Mac Pro when I'm not using it - I used to always leave it on and our electrical bill was about $280/mo. When I started sleeping it the bill dropped immediately to about $140. I have too much stuff to keep rebooting - Emacs, Xcode, 6 Terminal windows, a couple with several tabs, a CentOS5 64-bit VM for Linux development, an Ubuntu Server 9.04 instance (for building EC2 deployable .deb packages), openocd, usually lots of PDF docs up (mainly chip data sheets), screen(1)'s connected to USB/RS232 adapter used for consoles for embedded devices, frequently a WinXP instance running Bitscope software, OpenOCD, etc, etc. I'm loathe to reboot more than once a month even to install software updates that require a restart. So definitely a no on rebooting, but sleeping is an easy way to save a buck or two on utilities.