Hi stack,
A good quality tight fitting zip cover that will enclose all your layers is a good idea because it can help prevent the layers from shifting and it can also provide additional protection for the latex and keep it from oxidizing or breaking down prematurely (see post #12 here and the posts it links to).
The type of cover you choose can also have a significant effect on the feel and performance of a mattress and there is also more about quilted vs stretch knit covers in post #12 here and the posts it links to as well.
Phoenix