I never use osage as I live in the UK. I do use Elm, Ash, Oak, Rowan, Hazel and Elder mostly for my flat bows, have done for years. Took a break for over a year and was getting a bit rusty that is why I asked the question. With the woods I use, normally I can get a half decent floor tiller by roughly following the flames of the rings. Normally it gives a good starting thickness taper. Yes sometimes one side of the limb ends up thicker but if I slowly work with a file I can correct the thickness of the sides as I go because I work at glacial speed. I find it quite easy to stagger the rings to make a pleasing thickness taper?
To be honest I normally just stick the bow then on the tillering tree just to get my limbs to balance after floor tiller and normally my tiller does not take a lot of work as the floor tillering with the feathering has done the trick. It works for me but its only part of my skills to getting near a finished bow. The only I have to be careful is paying close attention to any knots which have their own way of messing with the growth rings on the belly. That is something that comes with experience and being able to read the run of the rings and taper accordingly.
I often find after adjusting the sides of my bows just using my finger calipers I hardly ever get limb twist anyhow. I can imagine the use of the rings like this does not work with all types of wood. I always free hand my bows and never use anything but a piece of string, axe, rasp and file so I can not really speak for how accurate my tillering is, the bows shoot and dont break so something must be right.
Thanks for the advice