J_F,
Airlines usually plan their flights on the 'minimum time track '. In certain areas of the world, such as the southeast section of Japan, this minimum time track is created by the convergence of two or three jet streams.
Eastbound flights to North America are routed into the track system just about where these jet streams meet and as PJ2 has mentioned there can be some bumpy rides associated with the minimum time track routes.
So, airlines don't look for bumpy air, but do look for minimum time tracks, which often mean bumpy air. :-))
I often took extra fuel on those eastbound flights and would occasionally drop down to the mid 20s to find smooth air if things were too bad up above. Another option if you were on the northern track in the system was to offset 25-50 miles north in an attempt to get on the northerly side of the tropopause. You of course lost some tailwind but the ride could improve significantly .
best regards,
Bruce Waddington