A foam pre-soak really isn't needed before every wash unless you have a good amount of dirt and debris built up on the exterior of the vehicle. I rarely use my foam lance unless I am stripping everything off the surface to detail a vehicle, as I typically don't use my power washer for maintenance washing. My mix ratio is typically 4-5 oz. and I usually mix in an oz. or so of Megs Super Degreaser.
I do on occasion however foam the car and actually just go over it with a wash mitt right away and do a wash that way. It just depends if I have my stuff out or not. This is actually a pretty quick way to wash a vehicle. Have a clean water rinse bucket to clean your wash mitt or sponge in as your going around the vehicle.