Macc,
I use whatever I happen to be using in my Trad bucket!
At the moment I have gone back to fairy, I only use a weak solutiion for window cleaning though.
I prefer to use the specialist detergents in a sprayer though, such as Unger liquid and so on.
Groundhog,
You have to bear in mind that you need a slightly different technique of cleaning the window, it'll also depend on your flow rate too.
As an example:
Yesterday I was doing a few shops in a local village, these come up flawless normally with WFP, however, when I went back to collect the sun was at that perfect angle that will highlight any flaws in your work.
Sure enough, on the doors of the one shop were some spots, ditto on another shop a little further along.
These were spots that I know have happened because of the brush being contaminated with unsoluable stuff....but on this occasion, and even though I had washed and cleaned the glass thoroughly, there were spots.......
Windows were rinsed with the brush
off the glass and not on it!!
Jobs were rectified (by trad methods) and the brush was cleaned, work afterwards was checked (just to be sure I was doing things right) and everything was back as it should be.

Just because you rinse off the glass does not mean you will never get spots, both methods work well, but to rinse on the glass your technique does need to change a little, practice makes perfect as they say.
Ian