Seems like this is a bug, but I don't know that I've ever tried to do this in the past.
I have an assembly, with two parts mated together such that their top surfaces are parallel but not coincident (1/8" apart). If I edit one of the components in-context, the measure tool gives incorrect results when checking the dimension between the two top surfaces. They are correctly reported as being parallel, but the normal distance is given as zero.
If the measure tool is used when not editing a part in-context, the proper value of 1/8" is returned as the normal distance.
Seems like a bug. I can envision all sorts of scenarios where this could cause problems. It is very obvious that the parts are not coincident in my assembly, but that may not always be the case.