If you use the BB spindle length suggested by the crankset manufacturer, you still may not end up with a correct chain line. Possibly if the crankset also makes a BB the odds for it being right on 42 might be better, but if you use another manufacturers BB it could be off.

I just installed a Stronglight 2000 track crankset that called for a 107MM BB on a 107MM Miche Primato BB and ended up with a 44.5 chainline. Possible asymmetrical BBs could also muck things up.