As we are all different players with different anatomy, it is a general rule (for R handed players) to stretch the R leg and slight bend the opposite leg.
Nevertheless situations occur where things turn out differently. For
instance with very tall people. Sometimes they prefer to bend both knees to lower their body position to the table. The same goes for players suffering neck problems or with older persons. They
to are all lowering their bodies to release tension in the curve of the neck. Is this wrong? Of course not!
Eventually it is all about adjusting to reduce tension and increasing comfort.