I freeze as soon as possible i.e. before marinading. It continues to marinate in the bag until frozen and again during the thawing process. Of course the beauty of having it in a bag means that you can leave it out without worrying about it or pop it in the fridge to continue the marinading process, without it stinking the fridge out.
I can only guess that perhaps marinating prior to freezing, would produce a slightly better result purely due to the changes that take place in the freezing and thawing process. But I am also not a Heston Blumenthal either.