Maintaining the correct valve clearance is important to prevent premature valve train wear. Too tight and there is a risk that the valves won’t seat properly and burn out, too loose and the valve train will hammer itself to death.
The latter happens because the clearance is taken up past the end of the opening ramp if there is too much clearance. The valves don’t open and close at steady speeds, but rather start slowly then accelerate hard, followed by a period of gradual deceleration as the valve approaches full lift. (Closing works backwards through this sequence) There is a part of the cam lobe, the opening ramp, which causes the initial lift speed to be low and constant-ish to allow the clearance to be taken up without hammering. There is also a closing ramp to avoid the valve crashing into its seat. (Valve bounce occurs when the cam is rotating so fast that the springs can’t keep the followers in contact with the cam so instead of being eased back on to the seat the valve hits it at high speed).
Ramps on original BMC cams usually finished at about 11 thou on the cam side, that’s 13.5 thou at the valves on std A series rockers, 15 thou on std B series rockers. After the ramp, acceleration is rapid so the further you are off the end the worse the effect gets.
If you have a sliding fit on the right size feeler but can’t get the next one in, you’ll be safely on the ramp provided your rockers don’t have any wear indentations where they contact the valve (most feelers are wider than the indentation and so bridge it, giving a false reading). If you have deep indentations here you should remove the rocker gear and get the rockers refaced.
I have generally set clearances on road cars a bit tighter than spec: one thou tighter on A series, 2 thou on B series (due to the official B series clearance being right at the end of the ramp) so that any gradual valve train wear (up to a couple of thou over the recommended interval) between adjustment checks won’t take it past the ramp. I have not observed any ill effects on valve or seat life on the engines I’ve had long term, including those with fast road cams and driven enthusiastically.
There can be a small performance upside of running extra clearance, but for longevity of the valve gear (especially the 1275 cam followers which require engine removal to change) I have always opened clearances a couple of thou just for specific events when every little bit helps then reset afterwards.
See also the page on valve clearances on modified camshafts.