I've repaired and built hunderds of computers, and this has been the
only situation I've ever encountered which caused multiple power
supply failures. While dirty AC is a worry, it's more likely to cause
damage to the MB than to the power supply.

Or, better yet, use better power supplies. Preferrably stuff from "PC Power
and Cooling," Enermax, or Antec. More expensive, but they're built to work and
built to last.

Another thing you can do is check out the weight of a power supply. This isn't
always an indicator of quality, but a heavier supply is bound to be more robust
than a supply of equal wattage but of significantly less weight. - Reinhart