Shutting off your computer, using it for work or racing is OK, just as long as you feel guilty when doing it. ;-)
As for optimized clients they do really help, and I have been replacing the standard clients as I get around to it.
My Athlon64 seems to be a bit of a puzzle as my self compiled clients both slowed it down, the 32 bit a small amount and the 64bit to a crawl. So I'm still using a Pentium 4 client.
I recently added another GB of RAM to the Athlon64 and also fired up the old Athlon 900 again. Adding the RAM improved the credit more than the old Athlon.