Sloped sill question

I'm replacing the windows in my house. The window sills are sloped. When I talked to the hardware store guy, he suggested using a chisel to level out where the replacement windows will sit so there is no slope directly under them. Is this a common thing to do? Or should I use shims to create a level place for the window to sit and then use some vinyl trim on the lower side (after insulating the resulting space)? Thanks.

No one I know would chisel the sill flat,, only if they made a mistake measuring and that was the last resort to get them to fit.
Some of the mfg have measuring guide lines. Take a look and see what you find.
Lets say your smallest sash opening heigth is 45 3/4", i would order the window at 45 1/2". That gives you just enough room to tilt the new unit into the opening. The window should come with a sill angle/adapter to fill in the outside difference of 5/8 to 1", I would foam or insulate this space before appling sill adaptor.

+1

Delaware Mikes recommendation on the other board holds true here are well as anthony's does.

Chiseling out a sill is not recommended.

Thanks. I thought that sounded like a wonky way to go about window replacement. Ordering windows today. Worked all weekend getting the outside trim and sills stripped and prepped for repainting. Only chiseling done was to remove some rotted sill to prep for patching.