Most motherboards should work without the -5V. It was used mostly by some specific sound cards so unless you have one of those you should be fine (and I believe there are adapters that give you -5V if you need it).

However, you will probably experience problems with old power hungry configurations (Athlon XP in particular) because those need strong +5V & +3.3V rails which are generally weaker on modern ATX power supplies, as mostly +12V is used nowadays...

You can get adapters that add the -5v to newer power supplies from eBay seller antronst.

They have them in every configuration possible.

You will want to use a minimum of a 300-350w newer power supply to make sure you have enough amperage on the 3.3v and 5v rails.

I've pretty much given up on old power supplies as well. The newer ones are so much better anyway. Higher efficiency, better regulation, better protection, power fault correction, etc.

This ebay seller was good information. Thank you. I will first test new power supplies without adapters and if not good, then try with adapters.