Gain Margin Rule of Thumb
From ControlTheoryPro.com

1 Gain Margin Rule of Thumb
The gain margin is one of the robustness measures of a closed loop system under control. A low gain margin can cause instability. Specifically, no measurement is perfect and therfore no model is perfect. When creating a LTI model from measured data there are always gain variations that are ignored as inconsequential. However, the lower the gain margin the more consequential those gain variations become.
The gain margin can be determined from a:
 Bode Plot of the open loop transfer function  magnitude at the frequency where the phase crosses 180 deg
 Nichols Plot of the open loop transfer function  magnitude at the points where the phase crosses 180 dB
1.1 Minimum Recommended Gain Margin
A good rule of thumb for the gain margin is
Application  Minimum Phase Margin (20*log10 dB)  

Tracking 
6 
Please feel free to add the recommended minimum phase margin for the applications you are familiar with.