Facing the BB is important for those bottom brackets where both cups have a flange, and they rest against the shell face when tighened to the proper torque - e.g shimano octalink, any external bottom bracket system. For those that are not this type - having coaxial bottom bracket threads is next similarily important - otherwise the BB binds when properly torqued. Due to sideloads on the bearings = e.g old campy square taper.

Proof is always in the installation, and comparison between non installed spin resistance an installed spin resistance.

Since there is never a sure answer, facing and chasing the BB always removes all doubt.