No at the beginning of Wrath Slow/Fast was NOT the way to go. There was a point where damage wasn't normalised and that meant that fast OH's became competitive, however within a couple of weeks Blizzard stomped on that, and normalised the damage of FT in particular. Some people who had already maxed out their Naxx gear swapped to a fast OH for those few weeks and swapped back when Blizzard changed it. However for the vast majority of players slow/slow has ALWAYS been the way to go.
Of course you had the sheep who changed to slow/fast because the top players were doing it completely failing to realise that their own gear was way behind the point at which slow/fast was viable so they just hurt their dps following the herd. It was for this reason that the cry "sim it for yourself" came to the fore.
This was repeated when haste became strong in late WotLK, there was a certain point at which it became better but it totally depended on your gear. The sheep went haste straight away and cried when their dps dropped or didn't increase as they expected. Of course it didn't because their gear wasn't good enough to gem/enchant for haste.
Moral of the story don't be a sheep, SIM IT FOR YOURSELF!!!