My installers told me not to fill in the dirt. They explained that the acid from the dirt will eat away the pool wall and rot it out prematurely. I have a slope where the pool is installed due to the amount of dirt that had to be removed.

Also, check the BOCA code. An above ground pool's wall is the barrier that stops people from falling in and it must be (or at least used to be at least 48" high) - this is one of the reasons why I chose a round pool. An oval pool with butresses would require a fence on the pool itself. My township was extremely nuts about the BOCA code.