I think I may have mentioned this here somewhere else, but for what it's worth -
A local building supplier, who has been in the industry since Moses played halfback for Dead Sea Scrollers, reckons that the grooves up or grooves down thing was originally a regionally based issue.
Apparently builders in Melbourne were installing the decking with grooves up because it stopped slippage around pools and spas, but builders in Sydney preferred the look of smooth decks. And he goes on to say that, if a company is milling timber for Sydney they will make sure the best face is the smooth side, whereas if the timber is destined for Melbourne they will mill it, so the best face is the face with the grooves in it.