The TR4 workshop manual says to run the valves every 6000 miles, but I've never done it that often unless there is a problem. Normally, the clearances will only change very slowly as long as the oil is kept clean and so on. So I only do it once a year, as part of what I think of as a "major tune-up".
In addition to excess noise as John mentioned, poor idle quality is another symptom of incorrect valve lash (particularly if they are too tight as a result of valve seat recession). Not getting a clear peak when adjusting mixture is another symptom.