This is most definitely how it should go.

It seems like an inevitability you will eventually have something go wrong (maybe you lose your group, your mask is a problem, you burn through air too quickly, etc). In this scenario the most important skill is to not panic and control your breathing but I think that mostly comes with experience.

While you develop experience, diving conservatively hedges against some of the worst case scenarios. Something as simple as a mask coming off can lead to panic which leads to hyperventilating. Someone hyperventilating will feel like they just can't get enough air and the immediate reaction will be to exit as quickly as possible.

Guides, as OP points out, should be far more sensitive to the experience a person has.