Do I have to tie my turkey legs together?
If you're following a particular recipe to the letter, and it specifies tying the legs together, then you might want to consider it. Otherwise, I wouldn't bother.

Trussing a bird will pull it together into a more compact shape, the reasoning for cooking being that it will cook more evenly if it's closer to a uniform spheroid shape, rather than having leg and wing bits sticking out all over. The argument against it is that it decreases total surface area, increases total cooking time, and causes the outside parts (particularly the white meat, which overcooks sooner) to cook more before the inner parts are done. The variations in your oven and in the turkey itself will cause just as much (or more) error as trussing.

That can be risky with stuffing, which will be the last to fully cook, and might have soaked up all those yummy raw turkey juices. Leaving the legs open (not to lewd) can give the stuffing a bit more surface area, and will expose the thin flaps of skin on the sides to allow even more heat to get to the stuffing sooner.

As far as keeping the stuffing inside, I doubt it will be a problem unless you have a very dry and loose or very wet stuffing - and if some spills out, so what? Extra crunchy bits.