The short answer to this is YES!
Even if you are going to allow your cat to be outdoors whenever it wants, you need to be sure that if ever there was a time when it couldn’t get outside for whatever reason, it’s already potty trained and can use the litter box whenever necessary. It may not be happy using the litter box, but you probably don’t care about that should you find yourself in a situation where allowing your kitten outside isn’t an option.
It could be that you need to be out of town for a few days and you don’t want to leave your cat door unlocked for security reasons, so your kitten needs to be kept indoors. It could be that it’s too cold, especially at night in the winter, so you want to keep him inside so he doesn’t come to harm in sub-zero temperatures. It’s even more possible that at some time in his life, he will need a surgical procedure that will result in him being kept indoors for at least 24 hours. If you have trained your kitten to use a litter box, the cat he grows into won’t have a problem if he finds himself locked inside with no means of getting out.
When you first bring your kitten home, have a litter box ready for him to use. You won’t be letting him out for those first days when he’s tiny, so use this time to “potty train” him. Use a shallow tray that he can easily get into and put a layer of kitty litter into it. As he grows, always have a clean litter box available, even if he doesn’t use it he’ll learn where it is and when the day comes that he finds himself lock inside, he’ll know where the “bathroom” is and how to use it.