The levels mentioned are hardly designer brand, just run of the mill tools that any half decent trade/diy would use, you also seem knowledgeable about how to keep checking your level, i rarely bother.Just fitted a cupboard with my Screwfixtastic level. All looks like it's the right way up to me.
I've also got tons of money, as I haven't been blowing it all on designer branded tools.
Don't forget to make sure your beans are upright