AS everyone else has said, don't worry too much, you are highly unlikely to scratch the sills.
On my brush - the Salmon one that Peter Fogwill supplies - the jets are stainless steel and protrude from the bottom of the brush, but were I to put the pressure needed for them to touch and scratch either the glass or the sills I'd probably break the window or damage the pole

Sills over time get grimy, and whether they are wooden painted ones or UPVC ones, there is nothing you can do to stop this.
Well, ok, you could use a non abrasive cream cleaner on them and keep them looking brand new, I've a customer that does that on a couple of her sills because her cat sits on them all the time :

Some weathering you cannot clean too well with WFP, but you will make marks in that weathering, they are not scratches, but merely lines in the grime.
Done trad this layer will stay there in a more even pattern, you wash the glass, you squeegee the glass, a quick wipe with a squeegee to remove excess water from the sill, and a quick wipe over with a damp scrim to finish.
I'll bet a pound to a penny that these sills are not on brand spanking new window frames?
If you see other windows on other houses on your round that look the same and you are worried, while the customer isn't looking, try a small patch with a proprietry cream cleaner (non abrasive!)
The sill will come up like new and you won't see a sign of the apparent scratches.
The next time a customer accuses you of scratching the sills, give them your equipment and ask them to try and scratch them with the brush.
Of course yo uhave to be careful that the scratches are not the result of plasterers getting cement on them and so on, but you can also clean a patch of sill with a cream cleaner and show them it is just weathering.
If they ask you to then clean the sills tell them ok, but it'll cost you £1.50 a sill
Also get out your trad gear and prove that you cannot get the sills any cleaner using that method (you won't, no matter how much elbow grease you put into it)
If there are very REAL scratches on the sills, the sort of scratches you can actually feel on your fingertips - I'm not talking about the odd one here and there - it is next to impossible for it to have been caused by you.
If there are red scuff marks, then those ARE probably yours because of the trim on the vikan brush, but that is just cosmetic and easily dealt with.
Ian