A disposal should never be used on a septic system and grease should not be put down the drain of any septic system. It's not so much they will fill the tank, it is the solids could be pushed into the leachfield and blind the soil surface to the point the system would not operate and maintenace would have to be required (read as a new leachfield installed). (At one time I was what you might call an expert on septic systems.)
As far as a disposal in the kitchen the waste should as much as possible be placed in the refuse. Grease as well. We tend to use it when things spill over in the sink and then run it to be sure the particulates are as small as possible before they enter the sewer system. My wife tends to put more down the disposal than I do. Something one should avoid in the disposal is egg shells, they really don't break down in the long run.
With all that in mind, I do like having them to be sure I never clog my drains in the kitchen with inadvertent waste entering the drain, but Martin is correct, do not use them as the main place to discard kitchen waste. (Yeah, in a previous life I designed WWTPs too.)
Phil, what I am talking about is to the right of the deck door and left of the cabinet. My guess would be a lightswitch.
The compost bucket can become a stinky thing. Think about that before you leave a hole in the countertop, since you will have to manage that waste and more than a few days in the compost pot can become nasty. You probably already compost and know that. I had a plan to put in a compost area in our kitchen, but ditched it. We don't generate enough compost waste since I am so rarely home until the weekend.