For my Green Iguana I just make sure she has what she must have first....i.e. collards, winter squashes, etc. as a staple...
As long as she eats her full portion of that, I give her treats of all kinds. Hasn't hurt her at all yet. but I am not giving treats out in the same amount of her staple food but Roxy gets borccoli once in a blue moon, a carrot once in a blue moon, wheat bread (ooohh boy watch out she loves it) a bite of my pizza crust, bananas, watermelon (which is water nothing else) and more....but just a bite here and there as a treat!
As long as you cover all the bases of nutrition, and your animal is a good eater of its staple foods, I feel giving something else as a treat is fine.
(well nothing harmful!)