Exhaust leaks will make the wideband show lean. Also, you may be OK at some Load/RPM's and lean at one or more Load/RPM's. It's the occasional lean spot that's gonna get you. Fuel and spark adders come into play too. They are based on ECT, ACT barometric readings load, RPM, Open Loop or Closed loop etc. The more things yo can adjust, the closer you can get your tune. Screwing around with just fuel pressure and timing ain't gonna make it. Today's good tune may kill you next week. OTOH, being able to muck around with your computer code can also give you a million and one ways to blow it up.