If you leave fuel in the tank for a long period it becomes like a sludge. At the earliest moment, drain your fuel tank, start your engine and burn away any fuel in the lines. Any unburnt fuel remaining in the engine can cause oxidization(rust) on any metal surfaces, eg. bearings, thus, shortening the life span of the engine.
People use after-run oils to try and prevent this from happening.
I would say that after-run is only required if you are going to store the engine for long periods, longer than a month.
If you are starting you engine regularly, once a week, or every couple of days, it should be ok.