If I understand correctly, the top of your cabinets won’t be flat when put together. You’re putting a heavy bench top on the cabinets. Why worry about it? It can be an inch out of level. Just shim the bench top.
You should see my bench which I plan to replace. I made it before I knew anything about anything. I saw something in the family handyman about a butcher block bench. I took a bunch of 2×4s that I salvaged from renovating and glued and screwed them together. No jointing, no special care except cutting off the rounded edge on one side of the boards. It’sa little warped and I set it on top of a bookcase that I reinforced and tied to the wall. I have an inch of shims on one side (partly to level it up off a pitched floor, partly because it’s warped). No one can see the shims without careful inspection and this sucker is not going anywhere until I decide to dismantle it.
The point is, don’t sweat 1/8 out of plumb for this job. It’s shop furniture and the only thing that really matters is the bench top. The rest of it (storage) is gravy.
Losing fingers since 1969