Testing appears to confirm what you said. My "C:\Windows\Temp" directory ballooned to 63GB on one of my computers, and 76GB (WTF, WINDOWS) on the other. Ridiculous. Microsoft's built-in disk cleanup utility was useless, saved me all of 3GB of space.
CCleaner be like, "hey yo, i'll save you like 80 of them gigabytes" and then I was like, "yea let's do that." So thanks, Piriform.