RE: to offset or not to offset
Try shooting them first with whatever fletching design you got. Naturally, if they shoot wierd or fly weird such as plaining or what not then you should try helical. Same concept as a spiraling football.
Also, if the blades of the broadhead seem to compete with the vanes, then a heavier broadhead which moves the balencing point of the arrow forward will give the fletching more leverage over the broadhead.
Nic