The biggest problem with deciding this issue is the lack of information about what officers did in the field. Even the photos from the LoC, while marvelous, are staged in their own way. First of all, they are taken at leisure, either long after a battle or before. It's clear some of them (from the tentage) are at HQ, where the need for a sidearm is usually pretty small (someone check their buttons and see if they're the "mushroom" variety indicating staff duty).
Even the photo of the jaunty three officers with their pistols and Hardee hats was not taken on the battlefield per see, but in front of what looks like a painted canvas backdrop provided by the photographer.
The better solution is to scour the writings of the period and see what, if any, conclusions can be drawn. The photographic record simply lags in time from what most LINE officers did in the field.
Proud Member of The Rowdy Pards
'In the end, it's the history, stupid. If you can't document it, forget about it. And no amount of tomfoolery can explain away anything that makes history (and living historians) look stupid and wrong."