I sleep my newer Macs because I use them so much. Often I need to use my e-mail when I'm doing tech support over the phone so I need them at my fingertips at all time. Also, I find OS X and OS 9 way too slow to boot up (though both are better than Windows).

I don't run old Macs all the time and only put them on when I need to in order to save the components.

I don't really care about how much power I use keeping the Macs in sleep mode. I need the comfort of convenience. I don't run A/C much at all in the summer and like the flourescent bulbs so I save some energy that way, which makes up for any difference from the average guy's electric bill.