Hey, Quicksand,
Just thought I'd offer my two cents (for whatever it's worth). I've been doing TKD for a while, and I've rarely seen a ball-of-the-foot strike with a roundhouse kick. I don't believe that this means that you can't or shouldn't use the ball of your foot to strike your target, just that it's somewhat rare. I suppose it depends on what it is you're planning on hitting. Certain softer areas (such as kidneys, throat, solar plexus, etc.) are more susceptible to a direct and focused attack, which the ball-of-the-foot method would probably be best for. The Instep might be better if you're striking an area where a pointed attack like the b-o-t-f strike might not be as effective (ribs, stomach, etc).
In addition, the instep is a slightly broader area, so the pressure of the strike would be more spread out, in terms of impact, on your own foot. A potential problem that might occur with using the balls of your feet would be if you don't pull back your toes enough... that would hurt a bit, if you strike a knee or a shin with a roundhouse kick's potential force, and your toes took all the impact. I've done some damage with errors like that. Granted, the instep poses its own problems for injuries, but I think it would be less likely.
Usually it's considered good form to strike with the instep, as it forces you to use control on your foot and strike with a specific point, which is good for overall technique. Then again, the same can be said for the b-o-t-f method. Up to you. I've also seen roundhouse kicks that utilize the shin to strike the target... although that can hurt and be dangerous if you don't know what you're doing. Ask your instructor, if you think he or she can help, and see what that person tells you is appropriate for your style... and start experimenting with it once you get that down. I'd think that would be the best practical method of getting the technique down.
At any rate, congratulations on the green belt. As always, the choice in how you utilize a technique, or what you do with the specifics of the technique, is up to you. Like everything, it's a matter of what you believe works best FOR YOU. Try different mechanisms for the technique, and do what you think is right for you. Just as long as you're aware that doing something different from everyone else in your style's probably going to cause some confusion.
Have fun, congrats again on the belt, and hope this helps somewhat!
Cheers!
--Cliarlaoch
PS: Recognize the name?
I'm on the server now, muahahah!
PPS: If you don't recognize the name... does the translation (cliarlaoch = warrior poet) help, or the name Chris ring a bell?
Thought so
Cheerio!