When calculating the Laplace transform of the derivative of a function, when do I approach f(0) from the positive half-axis?

Question

Reading my classical control theory class notes, my professor uses this formula to calculate the Laplace transform of the derivative:

$L[f(t)]=F(s) => L[f'(t)]=sF(s)-f(0^+)$

From the next class onward, he uses this:

$L[f(t)]=F(s) => L[f'(t)]=sF(s)-f(0^-)$

Doing some googling, I've found the latter formula used the most, so I sent him an e-mail and he replied something vague and heavily autocorrected about the impulse response. So, what I'd like to know is when do I use each formula?