For anyone travelling via Perth - Singapore route you can now pay $5 - 22.50 extra to get a definite seat allocation. Mine cost an extra $45 return to pick the seat i wanted - an aisle /exit seat.
Here is the website :
http://www.tigerairways.com/skylights/cgi-bin/skylights.cgi?module=C3
Fill in your flight details and then click on 'change itinerary'
A page will come up that shows you a picture of the plane seat layout. Click on the seat you want.