"in all the departure charts that I have seen in recent times I am not aware of any published procedure that says "Maintain Runway Heading" (I await the inevitable example!)."
Another one: if you look at the NOAA chart for the Oakland Five departure at KOAK (
www.myairplane.com has the one from December) it says "Climb via runway heading or as assigned..." This is the usual departure for aircraft destined north or east; I'd say fewer than 1% of departures are assigned a heading.