Potentially, more inputs being triggered means more things the cpu has to process, and less RNG craziness. It would probably be necessary to press the button at >4mhz to even potentially have an influence, but still an interesting theory and reason for doing it.
I personally start mashing right before the hammer expires, if there's a possibility of colliding with a barrel, but see that others do it religiously from the moment the hammer starts, and was also curious, aside from my half-baked theory above.