I think it is important to get it as level as possible. Even a 1-inch
difference will look funny when the water level is 6-inches from the top.
Also, a dif in depth will cause a difference in pressure from one side to
the other. If the depth is equal thente pressure is qual all around the the
pool is just stretched evenly. If pressureis lop-sided then the walls may
suffer. A small amount of slope should be OK but I'd try to make it zero so
the difference is actually small.

You can use packed sand but make sure that the weigh of the pool does not
cause it to squirt (slowly) out from the sides. There has to be some sort
of containmnt so the sand can't move sideways or upwards. You would have to
build a ringwall around the pool to hold about a foot of sand depth above
the pool bottom. The weight of sand would keep the bottom sand in place.