Ambient micing can really help give an instrument or vocal space in a mix. Depending on how you prefer the particular part to sit in the mix, will give you an idea of which mic to choose, how to position it and how much to blend into the close mic, if you are using one.
Ambient micing can aide in placing an instrument in the 3 dimensional space of a mix, front to back. Usually when something is far away it is muffled, more dynamic, and less distinguishable, the closer it gets to you, the more detail it has, the ear will naturally begin to compress and the dynamics become less by perception. Using this line of thought, you can apply it to your ambient mics. If you want the part to seem far off, then place the mic far away, even down a long corridor with little to no compression. If you want the part to be just set back from the immediate front of the mix, try the mic backed off a little, use a little more compression, and pan it just opposite the direct mic, this approach can give nice balance and space.
Ribbon mics can be a nice choice for loud sources, such as, drums, electric guitar, etc... But for other sources a multi-pattern tube or condenser mic is a great choice. Try an Oceanus or Clarion in omni-directional, this will help you pick up the whole room and usually provide a nice balanced ambience that you can blend in to taste.
Now there are really no rules and a lot of fun and experimentation can be had. Once you get a feel for your room and how the placement of the mic or mics in the room can really change depending on the distance to walls, corners, and floors, try adding compression, EQ, or filtering, to get something that really works well for you.