Mercedes-Benz tried really really hard to make it so only certified MB techs can work on their transmissions. That's why they designed their so called "sealed" transmission. The 722.6 (like yours) transmission is kinda a feat to simply check the fluid level. The 722.9 (new 7-speeds) transmissions are damn near impossible to simply check the fluid level, even for a certified MB shop. In your case, you really should have no need to check your transmission fluid level unless you find it leaking. So then, you simply repair the leak and correct the fluid level. A fluid and filter change never hurts. But it's not even an MB recommended maintainance on these newer transmission. But I don't think it's necessarily a bad thing to change it, say every 100k miles or so.
Did you ever check to see if maybe you did have a leak? It would only leak in two ways, really. Externally (with drops on the ground), or through the radiator/transmission cooler. You'd be able to tell if this is the case by popping open the coolant reservior and checking for traces of transmission fluid.