Yeah, testing different diodes is a favourite pastime among the stompbox crowd, but I’m not convinced all of it is that scientific 
Tweaking the level knobs might have more impact than switching between diodes.
The practical effect of assymetric clipping depends on the waveform, but there’s enough capacitors on the way to those diodes that a lot of stuff will be roughly symmetrical around 0 V.
(for a bit more on diodes as voltage-controlled resistors, see here and the rest of that thread.)