My key stays in the iginition switch on the tractor. That way I always know where it is and it is always available when I want to use the tractor. My JD 755 sat outside for 16 years with the key in it.
If you think by removing the key you are keeping a thief from stealing your tractor you are wrong. As others have posted, the keys are interchangeable and simple. It is also very easy to bypass the ignition switch on most tractors.
Case in point. A contractor was doing work at my house last fall with a JD 110 TLB. At the end of the day he dutifully lowered the FEL and set the backhoe bucket on the ground such that if anyone tried to pull the tractor forward (the only direction the tractor could be moved) the bucket would dig into the ground. He confidently stated that that ought to keep any one from stealing his tractor. He looked surprised when I said "yeah, unless they have a John Deere key." He tried his key in my 4310 and it worked. I tried my 4310 key in his 110 TLB and it worked.
It it is a good idea to remove the key from a safety stand point if you have children around.