When we say that in concave mirror 1/u + 1/v = 1/f

then here u, v, and f are absolute values and further derivation is done by assuming a particular ray diagram in which object is assumed to be at a particular position (though fortunately the final expression comes out to be same irrespective of various positions of an object) So, it is necessary to take care of sign convention while solving a numerical too. You can further have an insight for the explanation by solving the mirror or lens problem not by directly putting values of u, v and f in the formula taking care of sign convention, but by deriving the expression again using given numerical values taking care of sign convention. I hope that i am able to clarify your doubt.