I am in the process of making a quilt with needle turned gum leaves (65 of them) and the pattern suggested that the reverse side could be used with some fabrics to give a broader range.

If you like it, use it. The back is a mirror image of the front. Enjoy the results!

I just finished a quilt top in which I used the back side of one of my fabrics because it blended better with the other fabrics than the right side did. I have heard many times on the quilt shows: You pay for both sides of the fabrics so you should feel free to whichever side you choose.

Who said there was a wrong side or a right side, it's up to you.

Don't worry, I watched Fon's & Porter's quilting this morning, and the guess was a fabric designer. The quilt she was walking the audiance, had some slips of material sewn with the wrong side out. You are right she said the back worked to [mellow out] the bright colors of the quilt.
Why don't you make a pratice piece, maybe 18"x20" to see if it is something you may want to enlarge.

I don't think there's anything wrong with using the reverse side of the fabric. As most people know, some Hawaiian print fabrics are pretty bold and colorful and there are lots of Hawaiian made dresses or Aloha shirts sewn using the reverse side. They turn out very pretty. I imagine the using the reverse side of the fabric in a quilt could be very pretty, too.

Done it often. My most successful experience was using both sides in the same quilt - wrong side of the fabric provided a sort of shadow effect to the right side.
Try a pinwheel pattern, using both right and wrong sides of the fabric.
Experiment - possibilities are endless

Lots of us use the reverside of fabric, some intentionally and some by mistake. 8-) 8-)

I have quilted many quilts for clients that have used the backside of fabric. Sometimes the back works better!!!

I have used both the reverse side and right side of fabric all in the same quilt. It works great!!!!