About a week ago I transferred my betta to a 5.5 gallon tank, but this morning I noticed that the water level isn't even. He's already been in there awhile, and there seems to be no general problems as yet, but is this really bad? It's only a little less than a centimeter different.
All of my tanks lean, too. This is because they are on carpet. One tank has a pretty pronounced lean, but I just have to be careful not to bump it. You should be fine as long as the leaning isn't putting stress on the tank walls or base ie something isn't wedged under one side.