Not too important to have coated lenses in a viewer, unless they have multi-element air-spaced, lenses (unusual for viewer lenses) and then not even too important.  The most important thing is that they are achromatic (NO color aberrations), the FL is appropriate to the degree of magnification you want, the pair is well-matched, and you mount them in the correct orientation.  The recent issue of stereocopy was devoted to MF3D and lots of good info on viewers, FL's, etc.

-kp