Just to confirm, you tape the underside of the insole, not the sole of the boot?
Correct. Partly because tape doesn't adhere all that well to the bottom inside of my boots.
You can also tape the outsole (the bottom outside) of the boot - so that the blade mounting plates will fit snugly against the bottom outside of the boot. (But some boots, like Edeas, I think, place the part called the outsole inside a piece of fabric.)
BTW, some people like me have a a mismatch in the left-right tilt in the bottom of your feet (with relatively little weight on them) to the bottom of the boot - which leaves empty space that I think should always be filled, else you potentially lose control over what your boot is doing. In my case, it also caused pain, due to muscle over-use - because I originally tried to compensate by reshaping my feet to match the boot. You can also have a tendency of some feet (or ankles) to collapse to one side when weight is put on them. On top of that, it is possible that the shape of the sides of your boot put more pressure on one side of your feet or ankles than the other side - in which case you might want to make things more symmetric by sticking something on the light pressure side to balance forces. All of these things can affect the way you skate.