While reading Fine Homebuilding I saw an article where the builder used an angle grinder to shave off a little of the wood bucks to create a sloped window sill, rather than adding material or using a backdam. I have not seen this method before:
http://www.springtimebuilders.com/files/files/H254KN.pdf
http://www.finehomebuilding.com/how-to/video/weatherproof-window-install-prepping-the-rough-opening.aspx

Has anyone else done this before? I was thinking about doing this to the window bucks in an ICF house to prepare for the window install.

Sloping away is not a bad thing. If anything gets past the lower trim(also angled) it will at least be inclined to run back out.

I think if I was doing this however, I would stand the lower sill board on edge on a table saw and setting the desired angle rip the bevel before fitting the sill board to the wall. Once setup, I could probably prep enough lower sill for every opening in the house in the ammount of time he took to grind just one Since the boards/bucks are fit into the ICF once it is stacked, this method would also work well with wood buck material.

What blocks and bucks are you planning?

Ya the bucks are already installed in the ICF wall :-) as I didn't think about ripping the boards first so that's why I was thinking about the angle grinder.

I see... I hate it when my hindsight is 20/20... Havn't seen it done that way, but it obviously works in the video. A grinder or a sawsall. IF they are PT or plywood bucks, the sawsall with a coarse blade might work better as the wood treatment or plywood glue may tend to clog the grinding wheel...

Good Luck.

Good advice, thanks

Try a power plane for the main area and then use the grinder for the corners.

I have seen people use cedar siding. Already slopped and you don't need very much.