My Tiller tree had a curved block on the top, about a 2" radius.
I think it is important to support the bow where you hand will support it (a hand isn't rigid) and allow it to float.
(I know we dont shoot like olympic target archers, but look how loosely they hold a bow)
I even go to the trouble of winching the string back with a double hook on which the prongs are about 1" apart to simulate my fingers.
You will see quite a few pics of guys shooting where the grip is vertical in the hand at rest but at full draw it has pivoted, usually with the bottom of the grip pulling back towards the archer.
I'm not saying any of this is right or wrong, just that I do it so that as the string is drawn the grip stays as still as I can make it.
If you imagine a longrod (like those freaky target guys use) sticking out the front, you wouldn't expect it to move up or down as the bow is drawn.
Allowing the bow to move freely on the tiller gives you a fuller picture of what's happening as the string comes back...Ok I cheat a little to get the darned thing to sit square to start with by getting a bit of tension on the string as it will just topple over if it's not supported at the centre of gravity.
Hopefully you get my drift, (this is all more important on shorter bows)
Del