I always check actual TAS once established in the cruise. It's a fundamental part of navigating to me. Allied with groundspeed it enables me to get some idea of headwind/tailwind + trend. It's also an a/c health monitoring system. If achieved TAS doesn't match what the power tables say you should get then time to look for a culprit: Have you misplanned? How will that affect the rest of the flight? Is the weight different from expected? Leaned correctly? Does the engine have problem leaving it down on power eg compression low?