Yes, I think -12V and -5V is a hangover from ISA slot specs, but the question is since they are not being used, should they be rock steady on their spec values, or is it OK if they drift around or don't show at all ?
Is this indicating an imminent failure of the whole P/S, or intermittent spikes and brown-outs ?
I say again...if the system is stable, then i wouldn't worry about it. If you start seeing fluctuations in the +12V/5V/3.3V then start worrying. Although 10% is spec, I personally don't like anything more than 5% on any of those rails.
If you want to test your PSU, then run some heavy benchmarks and monitor voltages...within 5% and your PSU is still good.