Are there any rules of thumb for whether a concrete pad needs metal re-inforcement and if so, how much?
A simple rule of thumb for whether to include rebar or not is based on a 45 degree rule. In simple terms if the foundation is more than twice as wide as it is deep then you will probably need reinforcement, if it is not then you may be able to get away without because there will be little bending, and therefore tension in the concrete. 

There are of course a number of complications to this for example you might need to make sure that if the foundation does settle, it settles evenly. In this instance the foundation may need to provide some strength and so will need reinforcement. You will also need to be careful of large plastic cracks forming in the concrete at early age if you don't put reinforcement in it.