they say go parallel in general
some of the tougher guys go further... i recon its bs if you dont go at least parallel
some people do think going too low... like all the way down can be bad for you... some dont
as long as you're going parallel to the ground you're fine
heaps of guys at my gym stack up heavy weights and go 1/4ththe way down or 1/3rd... talk about cheating!