I wasn't trying to be vague with my response.
Several variables can create issues that Riggle and Deukster pointed out.
I can't recall ever applying a preemerge herbicide that wasn't watered in for two weeks in a row without some sort of precipitation. Since I am in the transition zone of Arkansas, the dew-frosts seem to deliver moisture........just a little extra kick to the 3 gallons of water per k that I apply.
I can assure that the density of the turf canopy, the thatch layer or rhizome layer can reduce the product from making it to the surface of the soil unless it is watered in. The labels usually call for .5 inches of rainfall or irrigation, otherwise the product is subject to light deterioration. If the product just lays there on the surface of the grass, thatch, then what else is it supposed to do. It must be water carried into the surface of the soil. We are in the cooler temperature days, and sometimes in the 60's but not hot enough to deplete the product too much. It is the over watering of the product that reduces the effectiveness..........therefore, no water reduces the effectiveness.
Since you didn't directly focus on a certain pre herbicide..........I am directing my answer to my use of Simazine, Prodiamine, Pendimethalin.