Boeings (737, at least) have an "ALT DISAGREE" indication if you are still in QNH if you pass the transition level on climb out. The reverse is true on descent.
Although, you're right - it is variable, but when you select SIDs STARs or approaches, it automatically inserts the expected transition level. Otherwise it defaults to FL180.
I wouldn't do it as you suggest, Freo, simply because the threat of leveling off at cruise in QNH is less than the threat of leveling off at the wrong FL because you didn't set it originally and leveling off mistakenly at transition level in mountainous areas could be deadly.