Is checking return values always required?

We know that checking return values prevent our software from unexpected states. (You can see CWE definition.) But, we are sometimes sure about the return value. For example:

bool calculateSquareRootReturnFalseIfInputIsNegative(float input, float& output);
float calculateHypotenuse(float a, float b){
  float c2 = (a*a) + (b*b);
  float c;
  calculateSquareRootReturnFalseIfInputIsNegative(c2, c);
  return c;
}

The local variable c2 is always positive. So, calculateSquareRootReturnFalseIfInputIsNegative() always returns true. I shouldn't check its return value.

Finally, Is "check all return values not already known by caller" a valid idiom?

Thanks..