I'm installing a tub in a 3 wall alcove which will be tiled. The manufacturer says to shim under the concrete board so it overlaps the tub flange smoothly. This seems like a good bit of shimming, especially since one wall is full bathroom length. This wall also has a window which comes very close to the edge of the shower/tub tile. I'm worried that if I shim the entire wall, my window extension jambs will not quite fit. If I only shim around the tub, I see how the drywall could transition back to no shims smoothly enough, but I still might have an extension jamb problem at the side near the tub.

I could easily notch each stud on two walls and fit the tub flange flush with the stud edge. This would mean I would have to shim even more the one wall on the side with the spout and valve as well as shim a little less behind the flange on this wall. In this case, it looks like I may also need to shim behind the valve so it sticks out a little more to connect with the handle.

Has anyone done this before? Are there other problems I'm not thinking about. Thanks

sure you can do that..
i would block between the studs. set the blocking back the correct depth that you want. this way you don't have to be fussy with the notches. then slide the tub in and the flange will contact the blocking and it won't matter if the notch is too big or too deep.
a few well placed screws and you are good to go..

Thanks for the blocking tip. Would you also drill and screw the bottom edge of the cement board through the tub flange to hit the wood blocking? This would allow me to secure it well at the edge, making a very solid place for the tile to rest... but is this inviting water issues. I plan to waterproof all the cement board with AquaDefense.

The wood blocking is installed with the 2x edge flat and the 4" or 6" facing into the room, so there is wood rising up above the top of the flange. Don't screw the board into the flange.

You can find ready-made shims. Might be easier and more uniform than notching.
Do a search under "drywall shims". Lowes and Home Despot have them for around 10 bucks/100 pack.

yeah, shimming is simpler but he said he had a problem with window jamb extensions and was trying to avoid that.
he should block the wall for strength so setting the blocks back 3/16 or so would be pretty simple.

There's a lot of ways to skin a cat. You're talking a 1/4" step. You could use a marble border tile down the raw end of the concrete board, and it would look like a design element. If the bathroom is gutted, now's the time to put the blocking in for towel racks, medicine cabinet, toilet paper holder. Have all those on hand and put the blocking in. Hoping you hit framing is not the way.