You pose such an excellent question :
Quote : But does one sell to lock in one's (substantial) profits now - and then re-buy in again at these (or perhaps slightly lower) levels? : End Quote
When a share is going up with strong volume, HOLD or even (as in MPO case) ADD, is my operating format.
No need to give that excellent institution the Australian Taxation Office it's due reward this 30th June. Make 'em wait! :)