My method is to watch air temperatures, soil temperatures and dates. Nighttime temps at or above 50 degrees F, soil temps above 60 degrees...I actually like to wait until the daytime soil temp is above 65 F. All this generally happens in my area after May 5-10. Our average last frost is the third week of April and I wait for a couple of weeks after that date in case we have a cold snap. Lots of early starters have to replant if they started in April, so why not wait until May. Later planted tomatoes catch up anyway and often surpass those affected by cooler temperatures.
And I never, ever prune. That I think sets the plants back in their natural growth, does not increase production or flavor and puts stress on fragile early plants.
