I wish I had levelers...but in reality I haven't had too many issues without. Only a few times where I've had to use blocks and fight with it.
That 3 degree/6 degree thing is the fridge manufacturer's spec
If you want to know more, check out this guy's web site.... even if you're not interested in his product, there's tons of info scattered throughout his site...have to do some digging to find all of it
https://www.arprv.com/