Greetings Paul:
Hardwood product markets are variable and some markets and products require longer log lengths. The key to bucking, of course, is maximizing lumber grade!
Crosstie markets might want 8'6" or 9', whereas lumber/veneer mills might want 8'6", 10's, 12's, 14's, or 16's. However, the most important point about bucking logs is "maximizing" lumber grade! This means, don't cut a defect-free 12' log to 8'6" just because it is a common log length -- the 12' log will likely be more valuable.
Please refer to the following Website for Hardwood Bucking/Grading Rules:
http://www.utextension.utk.edu/publications/pbfiles/PB1772.pdf
http://www.ces.purdue.edu/extmedia/FNR/FNR-190.pdf
Best of luck!