I believe it was at one time, at least I recall reading it from a reputable source. However, after doing some quick research on more current (Tiger & Leopard) stuff it looks like it may no longer be true. Furthermore, I just ran a quick experiment. I booted up 3 times. The first time was with an empty desktop, and I had 334MB used at boot. I then did it again. It was 340MB... so acceptably close in my opinion. I then put some pictures (about 300MB worth) onto my desktop and booted again. It was 335MB that time.
So, at least in Leopard, at least on my MacBook, it does appear to make any difference.