Most CAD systems allow you to adjust the soldermask aperture over (or under) size at CAM output.  This lets you change based on the design.  Some designs force you to use a smaller oversize, meaning you need to pay more for a fabricator that can do that for you.  Other designs have more room, you can use a larger opening, and a less expensive fabricator.

Again, I wouldn't recomend adding soldermask apertures to the CAD library.  There's a reason the CAD systems allow you to define them on the fly.

IPC-7351 really is a great "best case scenario" document.  But we rarely get to work in a "best case" world.

And don't let your fabricator change your artwork!  They have to make some process adjustments, but the final result in copper should match the gerbers you gave them.  Give them an inch........

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