It is only when the clearance is to a height that is below the TL that the clearance would be, for example, "XXX cleared descend to 5000 ft QNH 998" would be given.
If you are really still interested, as soon as a clearance below TL is given by ATC, then it is acceptable to immediately set QNH upon the heightimeters.
JW411, just one small correction to your erudite posting - for height read "altitude".
In aviation height and altitude are not the same thing - if you are flying on QFE the altimeter shows "height" if you are flying on QNH it shows "altitude"