Your partially right, but also partially wrong. Certain readiness tests are run at start up and others require a certain amount of time/miles to run them. SAI for instance is run on a cold start only, where as EVAP and catalyst require cold start and about 10 miles of driving. The 50 mile thing is generally what shops tell customers so they drive the cars long enough to set the monitors. Also the I/M monitors run differently for each manufacturer so what works to squeak a GM by might not work on other makes and vice versa.