Question 1174050
When there is confusion if one does not use them. 
Also use them when you might change the order of doing operations that appears to violate the rules. If you had 4+9/2, this would be 4+4.5 or 8.5. But you might really mean (4+9)/2, which is 6.5.  x-1/x is not understandable without parentheses; we see many problems written like this and have to guess what the person wants.  
Two minus signs together without parentheses may appear as one.  When I view the above, I connect the two minus signs and then put a vertical line through to make it +.  
To answer the question, a parenthesis is not required, but it does help clarify.  Don't drop the sign within.