It's standard practice to clear to your destination where there's a reasonable expectation that you will remain within the ATS route structure all the way.
As Gonzo says, your cleared on your own nav to follow your filed route. "Resume Own Navigation" or "Route Direct" to a point on your planned route puts you back on your own navigation, so you follow the flight plan thereafter. You should only continue on a heading if you have been previously instructed to - in the worst case simply following the QDM of the last received routing instruction might take you slap bang into terrain or a danger area, etc.