Yep, that's true. I just thought that is was interesting that the shape of a planet doesn't affect its trajectory under the laws of gravity alone (i.e. no collisions etc). It's also true that physicists do use explicit approximations often, for example in statistical mechanics, or by using a linear approximation instead of an exact quantity. That's similar to calculating the asymptotic running time of an algorithm instead of the exact number of steps it takes (except the limit in the physics case takes some quantity to zero to get a linear approximation whereas for the algorithm we take the limit of the input data size to infinity).