The most important factor in the routing of the diff pair in USB is the impedance. This is not related to the length but to the geometry of the traces wih respect to each other and the board.
A good reference on this for USB is done by Intel:
High Speed USB Design Guidelines
Excerpt:
3.4 High Speed USB Trace Length Matching
Use the following trace length matching guidelines.
High-speed USB signal pair traces should be trace-length matched. Max trace-length mismatch between High-speed USB signal pairs (such as , DM1 and DP1) should be no greater than 150 mils.
