I might add that if you are going to be bathing before you clip, ALWAYS brush the dog thoroughly BEFORE the bath, to remove all tangles and matting.
You should be able to run a comb through the dog's coat, all the way down to the skin, before bathing him.
If you cannot get a comb through the coat, you have not brushed him out well enough.
If you do not remove all matting and tangles before the bath, bathing will just tighten the mats and make them more difficult to remove.
Personally, if I'm going to be taking a lot of length off the coat, I will sometimes pre-clip before bathing.
If you are going to be giving your dog a shave-down (by shave down, I mean leaving a half an inch or less of hair), and he has a lot of coat that's going to be removed anyway, it saves drying time and shampoo, if you use a cut down blade to remove the majority of the hair BEFORE the bath.
Then, after he's dry, you can use a finishing blade to make the haircut look smooth and uniform.