With the introduction of the new feature which created a new voucher based on the remaining value, I bought in $500 lots last time. I'm yet to decide if I'll do the same this year: there were a few teething problems with this approach, and I still had to ring up each time to get codes removed to allow me to manage the booking online.
However, it does mean I can maximise the value of points earned and then redeemed against the vouchers without too much leakage of needing to pay more (I purchased 20 vouchers last year).
Something else to note, which I recently discussed with an agent: while it states that the expiry date of re-issued remaining value gift vouchers is governed by the original gift voucher purchase date, they seem to have no way to effectively track this when a ticket is issued. For example, an original voucher purchased on 24-Dec-12 would have an expiry date of 24-Dec-13. I then purchased a flight on 14-Jul-2013, and a new voucher was issued on that same date. When I received the email for the new voucher, it states Jul-2014 as the new expiry date on this voucher. The only date the agent could see against the new booking (which I needed to put into credit) was the ticket issue date: 14-Jul-2014. The only caveat to this, however, is that I notice when you go to redeem a voucher, it does show the original expiry date.
Fingers-crossed I'll never have to have the discussion, though I've always felt that if need be, I'd be able to produce the email from Qantas which lists the new expiry date of the re-issued voucher.