When using an angle grinder, should I use a different guard when cutting-off than the one used for grinding?


Yes, FEPA recommends that a guard, covering both sides of the wheel, is used for cutting-off, when using an angle grinder. When grinding, however, use an open guard. See drawings below for details.


Recommended for cutting-off


Recommended for grinding 

Warning: Never use a machine without a guard, as you would not be protected from debris, especially if the wheel breaks, you would be severely injured.