If a canvas is sagging are canvas keys the only option? I have only painted 3 paintings (at a Paint Nite). I can't imagine this would ever come up but if the canvas started to sag then I would consider installing some canvas keys. 

Does that work for all sagging canvases? I have a hard time thinking that would work for larger canvases. The canvas is just stapled to the frame so theoretically you could adjust the canvas by removing it and reinstalling it. That however seems like a bad idea. 

Perhaps this is just a matter of proper storage and display to mitigate this issue. 

Are canvas keys the only option with a canvas starts to sag?
No there are options.

Being able to use canvas keys depends very much on having the right type of stretcher bars to work with.

There are alternatives to help tighten a sagging canvas.

One method to help with sagging canvases is to spray the back surface of the canvas with a light spray of water. As the water evaporates the moistened canvas fibres will begin to constrict.