On the NG's (-800 at least) switching of the APU switch will trip the APU Generator of the buses and close the APU bleed valve. The APU itself will keep running for another 60 seconds....this then covers the one minute cooling/stabilising of the APU, which is a requirement according to my Systems Manual. After shutdown the APU Inlet Door will close in approximately 30 secs. So we leave the Battery Switch on for 90 seconds after switching the APU off. Still, although the APU bleed valve is closed as well automatically, you should still switch off the APU Bleed Air switch, especially before flight. Otherwise, should you need the APU inflight, the bleed valve will re-open, resulting in a Dual Bleed. Also we need to wait one minute after starting the APU before we can take bleed air, another reason to have the bleed switch OFF.
Myself I never heard of the rear service door restriction, but Ilitout's remarks make good sense, although in my company we frequently replenish catering through that door with the APU running.
Finally, a memo in our company recently stated the following: Boeing no longer has any technical objections against starting or shutting down the APU while refuelling. My company added to this that this is just Boeing, and that country, airport and/or fuelling company regulations might (read: will) still not allow this.
The other week my captain started the APU while refuelling resulting in a furious fueller.
Hope this helps.
Regards Z.