Hiya, You havent actually mentioned what colour your Stam is? If its a light colour then obviously your going to have to be very careful, if its darker like the black, brown or topaz it should be totally fine. You can spray appleguard on it, but on really light colours like the cream, it doesnt do much for it really.
I think the leather texture also makes a difference... if you got an icy finished leather then you don't really need to use any protectant. MJ bags are made to not require additional protectents, per the MJ boutique. Of course, IMO it never hurts to use one, as long as you test an area of the leather first.
I also would recommend cleaning your bag regularly, more frequently if it's a lighter color, and not to mention the nasty lil' beasties that are picked up along the bottom of the bag from use.
As with all bags, never put anything loose in your purse such as a pen or pencil or any kind of lipstick or make up. Keep everything in a protective pouch - also, carry a plastic garbage bag folded up inside your purse in case you get caught in a downpour!! I don't think the sprays do much but good leather cleaner when needed is a must.