I found that it does make a difference in the restriction with the downstream injector in line when running a surface cleaner.
I have QC's at the end of the jumper from the machine, at the hose reel and on the injectors so when I need to downstream I just add the injector and if I am going to use the surface cleaner then I will take the downstream injector out, you have to shut down the machine each time you take it out or put it back in.
I would not use a ball valve before the injector if you are going to use hot water as there will be a buildup of pressure in the injector, even when you release the pressure at the end of the hose. With the atmospheric pressure hitting the injector and the hot water, there could be a spray of hot water out of the injector and ball valve when you block the ball valve and take the injector out. If the ball valve is starting to leak by, you could be sprayed with scalding hot water, hot enough to take the skin off your hand and anywhere else it sprays. Be careful.
With cold water there is not as much of a risk but you have to worry about the ball valve going bad and leaking by, you could still get sprayed. Be careful.
I like the idea of a loop with ball valves and the injector on one side so you can open up the ball valve for the injector to be used or block it off and bypass it so there is no physically removing the injector so no chance of getting sprayed. I would use check valves if you made the loop, just in case.
