Does "sleep" slow down the system?

[Bob Baron]

I mentioned that I sensed the 'puter was a little slower to a 
(self-perceived) Mac guru.  I told him that the hard dry was about a 
third full and that I plan to buy 512mg more memory.  He asked if I 
powered down or put the computer to sleep when not in use.  I told 
him I always put it to sleep to save the time for booting up.  He 
said (paraphrase): "Aha, when you wake the computer it doesn't load 
everyting and it will make your computer run slower."  I've never 
heard this before.  Any truth to sleep mode slows computer?

iMac 17"
OSX 10.2.6
256mg
25gb memory used

Bob