I'm going to replace the windows in my 55 year old house. It's not a big
job - small house, one story. The window sills slope at about ten degrees. I
want to replace them and would like to know if it matters if the sills
slope. If I put in flat sills and make sure everything is caulked well,
should that work ok? The eaves of the house overhang several feet, so some
rain hits the windows, but they're partially shielded by the eaves. Thanks.

You really do want the slope. A flat sill will cause nothing but
trouble, no matter how well you caulk it. It isn't hard to do the
slope, will last, and will match the look of the house.

Is there some reason you don't want to do the slope?

-kevin

Yeah, I'm with kevin. You want some slope. No matter how great the
products you use or how well you prep/finish it, standing water will
kill a paint job. Better to shed that water -- even on a house with
brick facades, the brick sill is sloped.

Standard slope requirement is 15=BA.
If water gets into the wall assembly, rot, deterioraton and termites
follow.
TB