It's tricky so in the end I decided not to use flags. The issues are:
(a) in the future someone might add yet another MCAD format or some format such as PRC which can contain MCAD data; at the moment we have IGES, STEP, IDF
(b) Users may have a mix of models; component X may have a socket and an IC and each using a different 3D model format
(c) For complex projects It's a nuisance to flag every model to select one of "render,mcad export,both"
Personally I only use the models for mechanical checks so I never use VRML and people who are more interested in eyecandy would probably use VRML exclusively. As Maurice has pointed out, using his tools allows you to quickly switch between MCAD and visual (and all it costs is a little time spent specifying how to make a STEP model look pretty when it's exported to VRML).