Hi Ellwin,
Like DF/W I vary things a bit.
Firstly, it actually depends on what software you run regularly to some extent.
While programs are all MEANT to let go memory elegantly these days, not all of them do. If you find that your Mac is starting to slow up, and things aren't behaving quite as they should a restart is the first thing to try.
I find a shut down and "cold" startup (not just a "restart"), is not a bad idea every few days.
AS far as wear and tear goes, there are arguments both ways, but anyone who had one of the old "Summer 2000" iMac G3's (like me!) will tell you that at least on some drives , and if you have any sleep problems at all preventing the computer from sleeping "deeply", then bearing wear on HD's , resulting in annoying noise, can become an issue if they are left running continuously.
Its worth noting that these days the "sleep" process actually drops off power usage to pretty low levels, so some of the old arguments about leaving things running to avoid significant power and thermal "shock" to the system probably don't really apply any more. The change in such things when moving from deep sleep to full operation really isn't that different between the two approaches any more.
Regardless of whether you sleep or shut down your iMac at night, remember that the regular automated Unix "housekeeping" tasks won't be getting run unless the Mac is up and running, so use something like
"MacJanitor" or "Onyx" every week or so to do so instead.
Cheers
Rod