Scribes, Fillers, and Walls That Are Not Straight
Measure the wall before you trust it
No wall is flat. The only questions are how far out it runs and in which direction, and both are cheap to establish. Stretch a mason's line tight from one end of the run to the other, held off the surface at each end by identical 3/4 inch blocks. Walk a third block of the same thickness along behind the line every 12 to 16 inches. Where the block touches the string the wall is proud. Where a gap opens, the wall is hollow, and the width of that gap is the deviation at that point.
Typical readings on drywall over 2x4 framing: 1/8 inch of taping build up beside a stud, 1/4 inch at a badly bowed stud, and 3/8 to 1/2 inch across a long run where the framing itself crowned. Plaster over lath, still common in older Yakima housing, tends to be smoother in the field but throws a hard build up in the final 3 inches before an inside corner, which is what usually stops a finished end from closing.
Check plumb separately, because a wall can be dead flat and still lean. A 6 foot level held at three points along the run says whether the top is out from the bottom.
Scribing an end panel
A scribe is the honest fix for a finished end dying into a wall. Set the panel in its final position, plumb and at the correct projection, held off the wall by whatever the widest gap measured, plus a little. Open a compass to that widest gap exactly. Run the steel point down the wall with the pencil leg on the panel face, holding the tool square to the panel the whole way, so the pencil reproduces the wall contour offset by one constant amount.
Cut just outside the line, then back bevel the cut edge 5 to 10 degrees with a block plane or a belt sander. That bevel is the step people skip. Without it the panel bears on a full 3/4 inch of edge and any single high spot holds the entire panel off the wall. With it, only a knife edge makes contact, and that edge sands away in seconds until the panel sits tight.
Order end panels 1/2 to 3/4 inch wider than the finished dimension so there is material to give away.
Where a filler is correct, and where it is a cover up
A filler strip is correct when it does a job the cabinetry cannot: holding a door or a drawer front clear of a return wall so it can swing past 90 degrees, absorbing the leftover dimension between a fixed run and an appliance, or presenting a narrow surface to scribe into a corner. Used that way it is a designed component, cut from matching stock and finished like a panel.
It turns into a cover up when it hides something that should have been corrected: a strip disguising a run nobody shimmed straight, or a sliver closing a gap that opened because two boxes were drawn together out of plane. Taper is the tell. A correct filler is parallel unless it has been scribed to a wall, so a strip measuring 3/4 inch at the top and 1/4 inch at the bottom is reporting an installation error rather than a wall condition.
How much lean the doors will hide
Doors are the instrument that gives everything away, because they hang in pairs and the eye reads the gap between two parallel edges far more accurately than it reads whether either edge is truly upright. On a run built to 3 millimetre gaps, about 1 millimetre of difference between the top and bottom of a pair, roughly 1/32 of an inch, starts to register on anyone looking at it.
Work that number back to the wall. A pair of 30 inch doors showing that 1 millimetre of taper amounts to about 1/64 of an inch of lean per foot, which is far tighter than any wall will ever hand you. That is precisely why each carcass gets shimmed to plumb and to a common face plane on its own, before a fastener goes in. A wall out 1/2 inch over 10 feet costs nothing at the doors. A run allowed to follow that wall costs everything.