Hi,
Many times you'll see the offset voltage built right into the spice model of the op amp. Check for that first by a quick test of the op amp in simulation.
To add your own offsets, theoretically the offset can be positive or negative, and on either inverting or non inverting terminals. So to be sure you'd have to check with offset on either input and the offset should be of both polarities. Sometimes you can get an idea what the effect will be just by using a positive offset however.