I have a stock combo with slant dividers and I have found that if you don't tie some horses can get their heads over or under the divider to check out their neighbor - a recipe for disaster. If you are sure there is no danger in that, then leaving them untied is okay. Tied or not a horse/mule needs to be able to lower their head to cough etc. so make sure that if you use ties they are the right length for that horse - tall horse, longer tie.