Yes, we always pre-treat concrete unless it is a shopping center on a weekly basis or clean enough to not have to warrant the extra expense. Where you are located will dictate what you will pretreat the concrete with.
Always pretreat if you have dirty moldy concrete understand most of this concrete will be very neglected.Some customers wait 15 years an walk out one day and say wow my concrete needs cleaning.They call the local wash man an his job is to remove 15 years of yuck in 2hrs.Work smart an apply a very strong mix down an let it work a few more gallons of chemical cost will well be worth it when you make that first pass with the surface cleaner you will be a hero in the customers eyes.Good luck with your cleaning.
We do pre-treat sometimes on particularly bad driveways, patios, walkways etc. Usually if it's really grimy we'll DS our hw mix, wait ten minutes then clean. Makes surface cleaning a lot easier especially on mixed agg.
The pretreat with water idea is actually true. A lot of times it helps to just wet it and rinse the surface dirt and stuff real quick. If it's real bad worse case scenario (moldy mixed agg for instance) I'll pretreat with a strong roof mix via 12v. You'd be amazed how much that will clean in 5-10 mins dwell time.