If the lens has severe chromatic aberration, then red might focus at a different plane than other wavelengths, but it doesn't matter as long as you focus with the filter in place. With some older lenses it might be recommended to use a strong monochromatic filter all the time with B&W to correct for chromatic aberration.
The reason that IR is a problem is that you can't see IR, and most lenses aren't chromatically corrected for IR, so you have to adjust for the focus shift based on a calculation, and not what you see with your eyes.