Hi Dcsleeper,
You can read more about edge support in post #33 here but Jacob is right that most latex and most memory foam mattresses don’t have (or usually need) the same type of edge support that you will find in some innerspring mattresses.
In many cases … the feeling that you are rolling off the mattress may be an indication that the mattress is too soft for you or the comfort layers are too thick and soft for your heavier body type (and “roll together” in the middle may indicate the same thing) but this isn’t always the case and some people just prefer or need a firmer edge than others.
Firmer latex mattresses are less prone to this and there are also some latex layers that are made with slightly firmer edges (with pincores that are further from the edge of the layer or in some cases firmer materials are glued to the sides of the latex layers) which would help slightly as well but this type of construction for a latex mattress is not the norm.
If firmer edge support is an important issue then firmer layers may be a good option if you do decide on an all latex mattress or as Jacob mentioned an innerspring/latex hybrid where the innerspring has stronger edge support could also make a good choice.
Phoenix