Tapered or straight? Does it matter? How long do you like your foregrip and rear?
Depends On How High You Hold Your Rod During A Fight 
This is my personal opinion based on experience but i would go with 12'' to 14'' rear grip for a standup rod (rope or slick butt work the best),14''-18'' for a duel use rod (rope, shrink tube, or cork tape), and 20'' to 24 if your making a bridge rod (what ever you like). As for your fore grip a 12'' minimum to 16'' maximum gives enough room move your hold up the rod or open you grip up without straining your hands or arms to much. . With a good hand strength you will not slid back down the rod on straight or tapered EVA preventing you form having to constantly re adjust thus saving energy. This can also be achieved with sanding finger notches in straight or tapered EVA's. 