[Alan Anderson]

I've never tried closed-loop control of wheel speed. My use of PID for drive motors has always been position control for autonomous driving (and once for "hold position" mode when commanded by the driver). For tuning it, I set up a way to force the robot to drive a specified distance forward or backward on command, then tweaked the P and D parameters until it was repeatable within a tolerable error.

The "inertial load" on the wheels will change drastically when you put the robot down or pick it up off the floor. This will greatly change the D parameter for good tuning, and it is likely to have a significant effect on P as well, making the I parameter very important to compensate for small steady-state position errors. But for speed control instead of position control, you probably don't need to care about I.