Peter, using a float would be one approach, but I don't use them for M&T work myself. If it's necessary to clean up a tenon, for e.g., my trusty 78 rebate does a very quick, neat & controllable job on any tenon more than about 25mm wide. I do have a couple of float-like files that are occasionally handy round the shed, but I wouldn't really class them as absolutely essential tools. There are quite a few Liogier rasps I would buy ahead of their floats. However, we all have our own preferences & work methods, so it's a case of whatever floats your boat.... :D
Cheers,