Risers are a poor solution for taller riders because they move the bars back as they move them up. This pushes the rider farther back, which doesn't set well with the WR, as it likes the rider to be well forward when cornering. Taller riders have longer arms, and one of the things they find about standing on the bike with typical bars is that the grips are "in their lap", too close to their thighs to allow them to stand fully relaxed and upright comfortably.
The best solution is a pair of bars bent taller, because when taller bends are made, the hand grip part of the bar is moved forward, as well as up. My son, 6'4" likes the Windham (aka "RM Mid") bend, and so do I, in spite of being shorter than that. Other bends to look at are the Carmichael High, or the biggest of them, the KX High. The Windham is very popular among YZ riders.