What voltage should power requirements be listed as?
For 120 volt mains, it appears most list the requirements (amperage)
  relative to 115 volt. Should I do this as well? How about for 230 volt
  mains?


For CE (European) marking you have to specify the maximum rated current and, for universal input AC appliances, the maximum current will almost certainly be taken at the lowest AC voltage (maybe 85V). I am assuming of course that your device uses a SMPS to deliver low voltage DC to the functional electronic parts.

Think of the person buying it - they have to have some idea what the maximum current it could take. There's nothing wrong in specifying a whole range of stuff if you want to but don't foul up what the legislation asks you to put.

As per Winny's comment on the other post, go a bit higher to ensure every product can comply.