with stock cam timing i would bump ignition timing up about 2 degrees over stock. That seems to be the biggest gain. Not much else to be gained by going more usually. But like most car tuning it depends a lot on how you drive and how the engine is running.
On a stock cam Metro bumping the cam timing up 5-10 degrees advanced will get you up to ~5mpg if you adjust your driving style to match the lower shift points and don't drive fast. If you are a fast driver maybe 2-3 degrees advanced would show more gain since the top end will still have something left. The XFi cam is already pretty good so 2-3 degrees advanced is all it can really handle. The XFi cam at 4 degrees advanced can accelerate the car at 800rpm in 5th gear without a lot of effort. The car as nothing left over 3750 though.
A lot of this is mainly going to depend on driving style so there is no absolute answer. But in general advancing the cam to increase low end power so you can shift earlier and the torque peak is lower gains you mpg. The timing is really dependent on the car, gas, and driver/engine load.
Most cars are tuned from the factory on a steady state dyno so it is a pretty simple matter of making a perfect spark map that way. But the problem comes in when a car has to be sold in a lot of areas they have to cut some of the timing out so it is more tolerant of crappy gas. So in the case of an obd1 Metro the timing can be bumped up about 1-4 degrees depending on driving style without knocking and will get the timing much closer to ideal than stock timing will.
An actual vacuum gauge will give you a good indication of your cam/ignition timing changes. The smoother the needle without any vibration and the higher the vacuum the better.
http://www.amazon.com/Actron-CP7803-Vacuum-Pressure-Tester/dp/B0006V2BS2
This is the gauge I have mounted in my car and it is way better at detecting problems early than any other gauge you can buy. Everyone should have one stuck to their dash somewhere, they really are that handy to have