It is likely your flux field will build/collapse to different peak levels, depending on how heavy your chief load is.
If your chief load were to change, then that will cause different dynamics of the flux field. That is likely to affect the volt level coming from your auxiliary windings. (I'm not sure how much.)
Therefore it may be a good idea if you design the auxiliary windings to produce a much higher voltage than you need, to give you some headroom in case it drops. You'll also need to add couple volts, anyway, for regulating downward to your desired voltage.