Just looking quickly at the available downloads, it seems that, on Mac, ACR 7.1 was the last to support OSX 10.5, and 8.3 for 10.6.
On Windows, 8.3 supported XP (SP3) and Vista (SP2).
So, it might be worth us remembering this in future. This is a shame, because your advice has always held true (at least for Windows) until recently.