If a job consists of dirty windows - as in bird's muck, cobwebs, dirt etc. - then wfp will work fine if you spend enough time on a first clean. (and charge accordingly - of course)
To do it trad. would IMO introduce soap unnecessarily to the job.
If it is builders residue, paint, sealant etc. then only a scraper and solvents will do the trick - so you have to get up close and personal!