I'm making a 13 ft-long (4 m) built-in desk along one wall of a room. Adopting the engineer's creed that anything worth thinking is worth over-thinking, I used my neighbor's tripod-mounted optical level to make level marks on the wall. It turns out the floor slopes about 1/2" (13 mm) over the 13 ft.
Should I make the desk true level, or level with the floor?
1/2" isn't bad. I'd make it true level. You'd never install cabinets out of level I don't see why you'd make your built-in desk follow the floor.
Go with true level wherever you can, and in this case it's probably warranted too. If levelling the desk poses no optical distractions, then go ahead.
But there are cases where it might be better not to level: if you have existing cabinetry, windows, doors, trims etc.. where it would become noticeable that you have a 1/2in offset, you can go with the tilt or meet halfway for optical reasons.
Storing spheres, cylinders, card stacking and filling your coffee to the brim of course could forestall this option. Reminds of the carpenters' expression for level: "dead balls".
On the other hand, if levelling the desk highlights the tilt of other finishings in the room, then you could opt to apply some tilt or meeting half-way. It's not uncommon to apply gradual corrections when spacing out / levelling spindles, wall decorations, window openings, trims.
True level is the norm. Note that insurance assessors use this to determine how old damage is in the event of a claim.
Example: we had earthquakes in 2010-11 and our 1920's home sunk. The kitchen was replaced in around 2000. The assessors used fancy levels to prove that the kitchen bench was straight and the floor was not-straight, therefore the house had sunk before the kitchen was installed, and prior damage is not covered by the insurer.
UPSHOT: The benchtop and floor were tilted at the same rate-of-fall, proving to the insurer that the damage was not pre-existing and the house likely sank due to the quake.... but that was a decade of arguing and not relevant here.
