Well, there are scripts around, however, which are more "failsafe" than that, as they make sure that the composite DOF will actually cover the whole requested range. In other words, you won't risk having parts of the subject that are out-of-focus in every
exposure.This advanced script
, for instance, implements this feature.
It will also stop shooting when hyperfocal distance is reached, and sweeties like that.
If you prefer to write your own script or do the maths in your head, a rule of thumb that can be used is that if the far end of your current DOF is X, then focusing to X will still keep the previous in focus (at the near end of the DOF). It's not a mathematically hard rule as far as I know, mind, it just tends to work out ok IME.