I was wondering if someone knows a good way to keep the propellers ring on the mavic arms clean and how to check if there are after some fligth any obstruction inside, like grass or dirt. I had this issue after landing my drone in high grass and the back propellers slightly hit some of the stems before completely stoped. Once home i took off the propellers to check inside if the were anything inside. so my question is what are the chances to get obstructions and how accurate are the sensors inside that detect and obstrucion? Is it a good idea to check after a certain number of flight and maybe using a usb vacuum to clean them inside just to avoid any future problems?

It is never too needless to check the overall status of the drone no matter of its appearance or the app. For your case, I would suggest avoiding the dust from the source, avoid taking off or landing on a complex place. Check if there is anything came into the motors or the propellers, rotate the motor to see if it moves smoothly. Ship the drone back if you see something and you can't take it out.