If I left the tank with that level of unlevel it would constantly bother me, and I would level it. I really don't know if it would be a problem though.
One thing to consider is that on a normal tank it is quite possible that the top frame is not seated level on the glass, I don't think this is an issue on your tank which appears rimless, but I have had issues with that on one of my tanks. The bottom of the tank was level, the sides vertical yet the top wasn't level.