Follow the grain....Some staves may be fairly straight in appearance, when in fact the grain lines ...not so much. I draw a centerline, following the grain carefully tip to tip. My edge lines will also follow the grain on a straight taper, which may or may not match the centerline depending on knots or swirls and such. On your areas where you width taper, some violation of the grain is unavoidable. The angle of violation there is the key. 90 degrees is a break waiting to happen, so follow the grain in those areas, but cutting into the grain for a width taper, and the angle is tight, no biggie.
Different people get that taper done in various ways. I just measure it every so often and make sure I'm close. Too narrow is bad. A bit wide can be narrowed as you need it.