No it's not. I've been flashing 1.6 through 2.3, and this rule applies when the ROM you're using and the update you're installing are by the same developer for the same ROM. If the ROM is different, then a wipe is mandatory.
Oh btw, I actually saw an exception to this. There are a few ROMs out there that implement DATA2SD and DATA++, and I saw one there where the dev said a wipe is necessary even if you're installing an update. But this is just one ROM, and an exception.