3 different options for your kicking side hand during a round kick.
Hand drop -aggressively dropping the kick side hand to as a counter balance-
-Pro: correctly sending your hand opposite the direction of your kick makes the kick more powerful.
-Con: your hand is down, leaving a hole in your defenses. Especially dangerous for shorter kickers or when used verses taller opponents
Hand high -keeping your hands in 'fighting stance' throughout the kick-
-Pro: keeps your hands in a position that allows you to easily protect your head.
-Con: your kick is not as powerful as it could be, requiring slightly finer motor skill to land it exactly where it needs to land in order to be effective.
Hybrid -dropping the hand as you kick, but not allowing it to drop beyond your opponents striking tools and completely out of the fight-
-Pros: makes the kick more powerful than keeping the hand high, while allowing you to maintain a better defensive position than the hand drop.
-Cons: makes the kick less powerful than the hand drop, while leaving you more exposed than keeping the hand high.
I personally like all 3 options for different applications, and the hybrid for general use. I also don't consider round kicks to be a great option in self protection... But it's another tool in the skill-set toolbox, so better to have it sharp than neglected.