Hello fellow designs/horticulturist
This is actually something one of my clients asked me a week ago prior to a mulch job. I told them in a perfect world, I would use preen or Pre-Em, Snap shot w/e you use, prior to mulching, water it in, then mulch and lay down another application. Couple problems with this is we usually use preen w/ the fert in it, so a double application at the recomended rate would be using double the fert which I wouold be learing about. Im not sure what type of fert is in the preen, Ive never checked. But im guessing its not slow release and could potentially burn and plants taking it up if double the rate is applied. Another problem is this method would be pretty time consuming, and when you are charging $50+/ MH that would get expensive.
On almost 100% of the jobs we do that involve mulch and preen, we do it afterword and let the rain/customer water it in.