Depends on the way you construct Q. For half reaction

Q = [Oxidised]/[Reduced]

and Nernst equation gets form

E = E_{0} + RT/nF ln Q

Same goes for more complicated reactions, like

MnO_{4}^{-} + 8H^{+} + 5e^{-} -> Mn^{2+} + 4H_{2}O

Just put oxidized form in numerator:

Q = [MnO_{4}^{-}][H^{+}]^{8}/[Mn^{2+}]

(note that water concentration is taken from the Q, as it is assumed to be constant and not changing, thus it is moved to E_{0})

It is just a matter of convention used, you may as well use minus and reverse Q exchanging numerator and denominator.

Looks like I was not precise in my previous post, hopefully it is cleared now.