Hi blokes, it might be about the performance you hope for. I doubt a few thou would make much difference in either aspect.
Square four motors are expensive to rebuild, and you have to stop somewhere, but after all that money it would be nice to get performance that is as close to original as possible.
The followers could be as worn as the cam, so lift reduction at the valve might be double the numbers for your cam wear (more, if there is a clear ridge in the follower radius).
If you are going to have a cam reground, it's better to have it built up with something like stellite first, than to just grind the base circle away to regain lift. The followers may need doing too.
It's also worth measuring each lobe as a worn follower can cause damage to a lobe.
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.