E shreds magic resist so I would guess that hitting it early in a fight can help, especially a teamfight.You want to toss Q in after every cast to maximize damage. The W, R and E are interchangeable depending on the situation, but they should all follow a Q like Q-W-Q-R-Q-E-Q
Interestingly I just looked up Ryze's ratios and they're pretty low (0.2, 0.6, 0.35), though the 0.35 can go up if you get lots of bounces. Mana ratios are 0.08 and 0.05 for Q and W. I guess if you get enough money for AP items late game though, it's still worth it if you live long enough to get 6-8 spells off.