Usually, a bit more is allowed for scribing. It depends on the walls and how much material needs to be removed on the edge of the faceframe. Place a long level on the walls to determine how far out they are and add at least that amount to the desired finished size of the faceframe stiles. You leave one of the end stiles loose. Its not a bad idea to either rabbet or bevel the back side of the faceframe overhang towards the wall, so there is less material to cut when fitting. If this is a drywalled alcove, be aware that the build up of compound in the inside and outside corners often makes a slight curve from the corner out. Allowing only 1/4" of free play at the back of the cabinet might be too tight. You want the cabinet to have plenty of free room to slide in the opening.
Place the cabinet between the walls with the stile removed, level it up. Scribe the attached stile to the wall. If you allowed an extra 1/2" of scribe, use a small block cut 1/2" to use as a scribe. Pull the cabinet out and cut that stile to fit. Place the cabinet back in place. The loose stile is then held against it's wall and you make a small mark on the rails of the faceframe. Measure those top and bottom marks. If one mark is more than the other, mark the shorter one the same as the long one. These marks will be about 1/2" in from the end of the rails, if you allowed an extra 1/2" per side. Cut a small scribe block to match this measurement. Hold the loose faceframe stile on the long marks and scribe it to the wall. Attach the loose stile.
Scribing in a top is a bit more difficult since you have to fit to three walls. This method can be used on cabinets, too, instead of leaving a loose stile. Get some art matte board to use as a template. Use two pieces that overlap each other. Keeping the overlap square and straight, scribe to the back wall first. Then scribe one end to the side wall, keeping the back aligned properly. Slide the other piece to the other side wall and scribe that in. Carefully mark the alignment on the templates then transfer that to the work.