What kind of systems are you building?
-5V can be needed for some old sound cards, but there is a bigger issue when using a modern PSU for old systems, particularly the more demanding ones: since P4 and Athlon64, CPU (and PCIe) power has been delivered from the +12V line and nothing much comes from the +3.3V and +5V lines. So recent PSUs can deliver massive currents over one or two +12V rails, but relatively little on the rest. But prior to that, CPU and AGP power came primarily from the +5V line (and +12V was only for drives). With really old stuff, that doesn't matter too much, but with ATX systems, they might be dual CPU or late era AthlonXP. In that case you want lots of power on the +5V line, particularly if you intend to use a heavy AGP GPU.
Fortunately these are two birds you can usually kill with the same stone - PSUs with -5V also tend to deliver most of their power on the +5V line, PSUs without -5V invariably do so on the +12V line. I recently acquired a Fortron Source Power 350W PSU from the early 00's for this very purpose. Weighs a ton and can deliver 30A on +5V. No plans for a build that will truly challenge it, but it's good not to have to worry about it.