First few shots I had on big game animals I had time to range.
I fell into the trap thinking I was good at guessing range.. on my range. Where I shoot its flat and I get used to how big my target should look, as well as how much flat ground is inbetween it and I. However, throw in some small hills, replace dirt with taller grass, and change the distance of the trees in the backdrop, and it gets more difficult than you think.
Take the extra second to use your rangefinder if its obviously > 15 yards. If the deer/elk spooks in that amount of time, it likely would have spooked on your draw anyway.