Actually, if a saddle is truly rigged for a flank cinch, you ought to use one. However, since yours is an Aussie saddle, I don’t know if your saddle is actually rigged to depend on the rear cinch for proper fit or if it was just tossed on as an affectation.
For trail riding, especially in rough, hilly country, a flank cinch serves the very useful purpose of helping to keep your saddle in place. If I use a double rigged saddle, I use a “front” cinch instead of the plain leather flank cinch that you commonly see because it just seems to me that it will be more comfortable since I fasten it tightly.
Check out this older discussion: http://www.chronofhorse.com/forum/showthread.php?398069-Back-cinch-on-western-saddle where you will find this nice post from cowboymom:
A double rigged saddle without a functional back cinch is like a suitcase with only one side buckled.
The front cinch snugs the front of the saddle but the saddle is designed to have a back cinch stabilizing the back of the saddle, not just when a cow hits the end of the rope but when you’re trotting, turning, backing, walking down hills, trotting up hills, ect.
You’re not doing your horse any favors to skip the back cinch or to leave it flopping out there in the wind either. And ours are tighter than just touching, not as tight as the front cinch but not lightly touching either. It’s meant to secure the saddle; if it’s loose it’s like wearing sneakers with no shoelaces and it will sore up a horse.