whenever its not being unstable
sounds like a useless answer but really its the only one, components not only have varying accpetable temperatures but even within that some can be a bit more flaky than an identical one. Then consider that sensors detect temps in different places, in different ways, with varying levels of accuracy - and then the BIOS can have different ways & accuracy in reading them...
In short, temps are just barely comparable on indentical setups, and comparison is next to meaningless accross different platforms. That said, I consider it normal/fine for CPU to be under 55C under load, case to be under say 30C (heavily depends on where the sensor is though) and hard drives under, say, 30-35C?