It is important that the base for your tank is not only solid, but reasonably level for a number of reasons. 

Firstly, if you have your tank on a lean it may become unstable and fall over when full of water.

Secondly, regardless of whether the tank is level the water in it always will be - this means if your overflow is on the lower side of the tank it will overflow without filling the whole tank OR if your overflow is on the higher side the tank may fill beyond it's intended capacity, and there will be additional pressure from the water on the lowest point of the tank which may damage it.

Keeping these things in mind, the site does not need to be laser perfect but there should not be more than an inch drop over the foundation area in any direction.