You could (personal decision always) but I would never pass up a good opportunity to earn miles if I had not made it to the top tier status.
For this routing you can probably get a 700 GBP fare (LHR-LAX-SYD) on NZ in econo and earn a whack of miles. Do 4 (change noted below) of these and you could be UA 1K with 6 system wide upgrades. Then you could upgrade this kind of itinerary LHR-LAX-SYD with one upgrade certificate. Maybe not as cushy as QF but something to think about. Don't know what the upgrade deals are with AA/QF but QF has even cheaper fares. Consolidators might do even better.
If you have a burning desire to use rather than earn points, then it is always best (IMHO) to use them on business or first. You will probably be happier spending the extra 15K on QF than flying UA but lots of people fly UA in business and find the service acceptable (not a ringing endorsement I know).
[This message has been edited by BlondeBomber (edited 01-28-2001).]
[This message has been edited by BlondeBomber (edited 01-28-2001).]