installing base cabinets
Before you start installing the cabinets, you need to find the high point on the floor and create a level line around the room from that. You then shim up to the line. If they're are any gaps in the back of the cabinets, you need to shim those too. If you screw the cabinets in, pulling them tight to the wall, it will distort the cabinet.
If you find the wall is less then 90 degrees you will need to pull out the corner cabinet enough so the front face frames line up.
You will find out when installing cabinets, walls and floors are not exactly plumb and level.
It's more fun with the wall cabinets.
In either case, the corner cabinet is the key.
"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety."
B. Franklin 1759