I haven't seen this covered previously, so here goes. WE (emphasis mine) add our surfactant BEFORE we leave for a job, with the tank filled. The slight agitation from driving does enough to mix it without causing it to over-suds. Back in the early days, we used to add it on the job site, then spray the water in, but this caused too many air pockets, and didn't work as well for us. The agitation seems to overdo things just a bit, so we always add it at the house, then let the potholes and crappy roadways take care of the rest.
Good Idea, we do the same. We actually STIR at the office, and let the roads take care of the rest. We also VARY the amount of Soap according to the STEEPNESS of the roofs we have to do that day. Steep Roofs = More Soap = More Cling