when do we indicate the "+ or -" sign in uncertainty
You either use both "+" and "-", $\pm$, or neither in which case it is implied that you mean $\pm$.

Usually uncertainty is expressed in a way that is symmetric about the mean, such as $123\pm4$, meaning to a certain degree of confidence the true value is between 119 and 127. 

However, you could write $123^{+5}_{-3}$ to indicate the best estimate of the true value is 123 and to a certain degree of confidence the true value is between 120 and 128.

For example, if you were highly certain that an object is a cube, and measure its edge as 100 $\pm 20$, then it might be better to express the volume of the cube as 

$1.0^{+0.7}_{-0.5} \times 10^6$