MrsRupert,
It used to be the case that within the US, flying one way would subject you to much more scrutiny than otherwise. That would be one of the things that would be likely to get you flagged for further screening, for example. However, this is much less the case now since more people fly this way cause of the way airlines charge for one-way vs roundtrip tickets within the US nowadays.
As for one-way international, well I've flown one way NY-LON numerous times, most recently in March, and have not been bothered about it ever.
Honestly, the only thing I'd consider in your scenario is the price, as someone mentioned above. It isn't unusual for a RT ticket to cost less than the OW.