I am going to replace my square tapered spindle type bottom bracket with a Hollowtech 2 external bearing one.

In its simplest form, I guess this will involve removing the old one and just thread in the new one - but I've seen some articles that suggests that the bottom bracket shell faces must be machined.

Is this "in rare cases" or should the shell faces always be machined before fitting an external bearing one? Even if the frame is not new?