My Scrabble group (alas, back in the days when people gathered in "groups") used the official Scrabble dictionary for challenges. We also allowed people to browse through the dictionary when it was somebody else's turn, but not when their own turn came. That was our own rule, sort of a compromise, as some players wanted to have access to the dictionary all the time, which slows the game down enormously, and is really not the way it is meant to be be played.
One interesting thing is all the different editions of the official Scrabble dictionary. They seem to come out wtih a new one every year. So, if you have an older edition, words like Google, Blog, Skype, etc. are not in it. But they are in the newer ones.
One word that I tried and turned out to be not accepted is "twat." I always thought it just meant an annoying person, but it has another meaning. I always imagine a committee of prissy old biddies working for Scrabble, voting yes or no for some of these words. Words With Friends is even worse.