Good point, of course. Everybody is different, and one must adjust methods accordingly. I would argue, then, that if one left the toes untied, one still has the option of bending the toes back to stretch out the soles. If you tie the toes and want to tickle under, you're out of luck.
On the other hand, bending the toes back manually occupies five whole fingers otherwise put to more active use. Maximum overdrive is best achieved when all ten fingers are going at it.
This is all academic, by the way. The soles/arches are a close personal second, at any rate; it's not as if no meeting of minds is possible. 😛 We ticklers have a way of getting five feet up the bull's ass of our own ideal... setups... scenarios... whatever (I've heard interesting stories regarding the truth about hogties). We shouldn't let that get in the way of the reality on the ground.
In sum, I defer. Mostly. 😛