I just went to fit the taps to a new bath but when the nut is tight the
taps will rock for/aft; presumably the underside of the bath top has a
slope. Is there a standard way of dealing with this without having to
faff about with homemade wedges?

Dave

Dont faff around with lead washers etc. If the slope is on the
underside of the bath make a spacer by finding a piece of pipe that
will fit over the tap threads. Cut one end to suit the slope of the
bath and the other end square. Dont make it too long. About half the
length of the threaded portion will do. Tighten all in place using
rubber washers.