Which voltage for custom transformers in USA; 115V, 120V or neither?

I'm about to order a few custom wound transformers from an European manufacturer, I am in the US. I have seen 115V+115V as a rule of thumb everywhere, but if these transformers are intended for use in the US, which primary windings should I spec? I initially quoted for 115+115V, but 120+120V would be a better option?
 
I often see 122V but it ranges from 117 to 125V.
All the recent NEC and UL documents list 125V.

Power transformers don't like to be over-voltaged. Many of the DC blocker threads are about problems caused by high line voltage rather than DC offset.