If you are cleaning your regular accounts, you will already have got the sills clean, the water flow from your brush, coupled with a quick wipe is usually more than enough to keep them clean.
And depending on the angle you have your brush set at (I use the Unger setup and its completely adjustable) even on upstairs windows you can still wash the sills.
On some downstairs windows, as has been said, muck can gatehr in the gap between the frame and the sill and you keep flush the bloody stuff out!
On some I have spent extra time, kept the water flowing and used a bit of stick to run back and forth in t he groove so that I can flush out the cr
ap.
On subsequent cleans it makes a big difference.
Not so effective on patio doors though as the sill is at ground level
and the groove is always getting filled up with detrius!
Sometimes I'll just use the flat of my hand to run over the sill, that works too.
Regards,
Ian