Electrical usage is minimal when sleeping but it all adds up. The cumulative cost of wasting energy attributed to electronic devices that are always on is considerable.
There is really no reason to leave a computer running 24/7 unless it is actually being used as a server or there is a warm body in front of it...
Macs benefit from being shut down on a regular basis. Depending on how it is being used a Mac can slow down within a couple of days due to RAM getting tied-up. Shutting it off regularly prevents this from becoming a problem.
An alternative to leaving it on sleep for extended periods of time is to use the Energy Saver preference pane to have the Mac turn itself on and off so that it is ready to use when you usually begin using it during the day and shut down at the end of the day.