From what I gather, seafoam will remove very mild varnish and crud from carbs. However, as with adding it to oil, you might want to be concerned about large chunks of crud that get loosened up and relocated. When I was on the Kawasaki Vulcan 750 forum, a lot of guys would add it to the gasoline every other tank with good, but anectdotal, results. Our carbs are so much easier to remove than a V-twin's, so like k_enn does, I'd just use the Seafoam to control moisture. The best way to clean a carb is to just pull it and clean it.
If you do add Seafoam or MMO to your oil, make sure you change the filter after draining. You want all that stuff out of there, as it thins the oil and the filter might get crudded up too.
My [smiley=2cents.gif] is to just maintain regular oil changes and go to synthetic if you want to keep an engine squeeky clean inside. Synthetic oil has a way of finding leaks and, as I understand it, more detergents to keep sludge from building up. Be warned though, cleaning carbon deposits from around the rings on an old engine with some mileage might result in burning oil. I have a '77 Mercedes 240 Diesel, and after running it on diesel purge (a serious injector cleaner that's run out of an auxiliary fuel tank for 5-15 mins.), it started burning more oil.
Furthermore, I would not ever use any type of solvent or synthetic oil in a high mileage motor with hydraulic valves (not to worry on our airheads), as you can end up with crud blocking the screens that keep junk out of the hydraulic valve chambers, and then your valves dont open enough
Lastly, it's usually good to reduce the tire pressure to prevent flat spots in the tires that develop over long term storage. You also might be able to put the bike on the center stand and block the motor front and rear, so that neither tire is touching the ground.