VW

I would re-phrase the question. "Could the +12 be allowed to vary over a range of +7 to +18 without affecting the operation of the mobo. My fear is that each mobo will have a different answer.

PHC
I totally agree with the need for switching regulators. There are some that are very cheap & simple & efficient. Speaking of cheap and simple, here's the 12v switching regulator for Lilliput (as I'm sure you've seen). Looks like about 7 components. It uses an LM2596. You could use something similar for +5 & +3.3.