A lot of great reply's but I'll put in my .02 so that you can poll the answer with all the other reply's.
MMO: I run a very little bit of MMO in my gas if I am forced to purchase gas with ethanol in it. Specially if the engine will be sitting for any period of time.
Seafoam: Seafoam is a treatment and is cleaner, I do not add seafoam to my engine oil or to my fuel. Seafoam is great from removing carbon and I find it works best when you flood the intake system with it as per the instructions are written.
There is no need for oil additives other then zinc to your engine oil if you keep your oil changed your engine will stay clean , the only build up in the engine that cannot be avoided is carbon and carbon comes from use and or bad gas or or even blow by of engine oil. I bet the MMO also put a few deposits in there as well, so every blue moon or so or when I feel the engine has seen alot of hours I'll give it a good wash down with some seafoam to clean it up.
I will also seafoam on unknown to me engine before I start working on or repair it, I have had one that was sputtering and popping through the exhaust pretty bad and then I did a few seafoam washes and cleared it up completely so I find that straight up is the best way to use it.
If a carburetor is gummed up ,I take it apart and clean it. Because even if it breaks up that crap in the float bowl where is it going to go?