USB differential pair length

While the length and impedance are both important, 1mm of length differential will not affect your system's performance in any way, even for usb-2.0 high-speed.

From the USB spec:

7.1.3 Cable Skew
The maximum skew introduced by the cable between the differential signaling pair (i.e., D+ and D- (TSKEW)) must be less than 100 ps and is measured as described in Section 6.7.

Assuming a perfect propagation velocity (i.e. C, the speed of light), a differential length of ~2.99 cm would produce a skew of 100 ps. As such, your 1 mm of trace length differential will not be a problem.

Added: On a real PCB, your signals travel slower than speed of light. For a stripline (inner layer) you divide the speed of light in vacuum by the square root of the relative dielectric constant (e_r). So about half speed. This means the 100ps is more like 15mm. For the outer layers, the speed is slightly higher (about 10%).

It isn't the distance, per se, that matters. It is the impedance of the strip line or microstrip that matters. Use any calculator in your CAD software or online to get 90 ohm differential. The impedance depends on the spacing of the traces and their height above the ground plane. An sample calculator is on eeweb.

1 mm difference in is fine unless you are doing SuperSpeed USB 3.0.

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.