My motherboard reports rails that all dip down near uncomfortable levels, especially 3.3, even with different PSU's, but a DMM proves otherwise. For example, mobo says 3.3 fluctuates between 3.05 and 3.24. DMM says 3.32 solid.
Moral: if you're really worried, get a DMM if you don't have one already and
check this out.
AFAIK the negative rails aren't really used any more so they are not important. Many PSU's don't even have them these days (might've been removed from the ATX spec).