If it is a stiff handle, in other words, the design does not allow the handle to bend, the shape of the handle should not be a factor in tillering. Advance bowbuilders may consider it but don't complicate your life right now. Make it straight.
After the bow is actually a bow, in other words, you have shot it a bunch at full draw, shape the handle to your liking without removing enough wood so that it will bend.
If you make it appreciably thinner or narrower, it may start bending there. That is generally not good.
Selfbows traditionally do not have contoured, locater style handle found on modern glass bows.