I have two topless tanks one has a Picasso in it an not a problem yet. I love the look of topless tanks very clean looking and one less thing to clean water spots and salt creep off of...the money I saved by not getting the canopy/lid I put towards my ATO budget and bought a very nice ATO because I do think it evaporates faster topless.
One thing though, if you have a cat or anyother jumping curious animal be careful...when I was a kid my Dads cat would sit on my Moms glass topped 55gal fresh tank for hours at a time and watch the fish (Dad had a heavier glass cut for the lid to hold the cats weight) a time or two someone forgot to close the lid after feeding, he was always okay but you could tell he wasn't happy with us for the few days after it would happen.