My preference is for the serial and model (from somewhere around 68/69 on) references to be intact, on those models that had them (it's worth being aware that there is some variation by model as to where and if the case is stamped). The references being legible is also nice, but doesn't always happen after 40-odd years - if some of the numbers are obscured, I would prefer it to be from general wear and tear than polishing.

Having the serial helps with record keeping, but is also a useful check in making sure the serial is correct for the period of the watch and thus originality - basically a rough test for the watch having been assembled from parts. If I was offered a watch that would normally have references but apparently never did, then I would be looking closely for other signs of it being a put together and for that to be taken into account when negotiating price.