squealing sound from rear brake

2002 4-door Honda Accord has 80,000 miles, replaced font brake and rotors with 10K miles,
Right side rear brake drum makes high pitch sound whenever I release the brake pedal.
Problem can be reproduced at the following steps:

1) Park the car, turn off the engine.
2) Step on brake,
3) Release the pedal, squealing sound from rear brake.


I'd like to know what cause this problem, should I replace the rear brake shoe, drum?
Is this a safety problem? What will happen if I ignore this problem, any damage to the car?
Another question, unlike Disc brake with wear indictor, how do I know if it is a time to change Brake shoe or drum?

Thanks for the help