Most things are personal preference
Here's how I look at it - hypo is a great cleaner/killer of organic material like moss,,algae and lichen- it is not very good for dirt,debris and non organic stuff like pollution and dirt - if the surface is covered in the latter applying hypo will work but you need more of it to cut through stuff that is best removed by the p/w - also to my mind you now have hypo all over the surface which you now blast with a p/w sending hypo over nearby grass, plants cars, clothing etc...
By applying after you can see exactly where you need a strong dose and lightly and acurety spray the entire area giving the hypo the best chance of doing its job
Darran