[gpetilli]

For positional control, yes start with P and raise till unstable - back off and add some I to achieve closer final value.

For Velocity (Speed) control, many teams have had better success starting with I and adding P. The issue is with velocity, a negative proportional error will try to drive the motor backwards. The other way to solve this is to use a feed-forward term, which is not available on the Jaguar.