Suppose its all down to the individual collector. If they want certain depictions in dioramas then they can really do whatever their imagination lets them.
I suppose if you are mixing and matching vehicles from early blitzkrieg with say Kursk some may say its wrong historically, but, as long as the individual collector is happy!!! thats what counts in the hobby.
I don't mind if the AFV has no unit markings as not all had them some did some did not the new panther is a good example as it allows collectors to put it in any setting with any troops, italy, normandy, eastern front so, stick with regiment numbers and limit the unit symbols which, will allow collectors to use the peices without fear of being castigated for incorrect use. IMO
Mitch