wfp or trad?
Actually I assume wfp.
If theyre new then you shouldnt have a problem with them, as they shouldnt leak from between the 2 sashes and shouldnt also be covered in cobwebs.
If you want to be sure you do a good job, do the top sash first on all the upstairs windows and then go along and do the bottom sash of the up stairs windows, starting with the first that you did and continue to do them in the order that you did the top sash in. Rinse on the glass to help to prevent splashes contaminating the glass from the runners etc where cobwebs, dust etc can collect.
Then go along and do the same on the downstairs ones.