I think it makes sense to prevent any form of overlapping within the absolute tolerance.
However, in my experience there is only a handful examples where it doesn’t matter how big the underlying surface is. In practice the underlying surface is part of a bigger surface layout, and as such it makes sense of not touching it, especially just for trimming.
The problem usually is not that the surface is touching the cutting curve. Instead, often the trimming curve is not fully on the surface, resulting in a very heavy trim boundary curve or going beyond the surface border. The projection vector is important then.
Also note, you cannot truly extrapolate all surfaces without the risk of modifying continuity or stability. If you linear extend a single- or low-span surface you shouldn’t run into any problems. This is very stable, but consider a heavy surface. Usually what happens, you also extrapolate the error. Very un-smooth surfaces can easily go wild. So there is a lot of things which still can go wrong, even if you prevent a tolerance issue.