As long as the handle isn't bending removing wood from the handle area won't affect tiller. I usually go with about 1" wide at the arrow pass on the bulbous handles I use.
it seems to bend ever so little,....I noticed that right away when I started tillering it. It's the same now, ( I just strung it and held the handle fast in my palm, pulled a few times.
It's not terribly noticeable though, so who knows....maybe its the fades giving that I feel just starting to go into the handle area as the handle is a wee bit under 4" long.
Think I'm safe to have a go at it, (cut a shelf in or narrow it up more, it's currently at 1 1/4" wide)... or should I just err on the side of caution and leave it as is?