Heavier broadheads will affect the spine of the arrow but does not nessessiarily mean you have to change arrows, now if you are already usung a weak arrow and go up in point weight you will most definately experience underspined arrow.

most important thing Imo is not to be too heavy on front because a lot of FOC% will cause the arrow to drop much faster
opposed to an arrow that is between 7 and 10% FOC.

unless you are hunting Big affrican Game I see absolutley no need to use any thing heavier than 85 Grain. JMO