The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types. [zero, positive, negative]
more info here:
http://www.tirerack.com/wheels/tech/...jsp?techid=101
i don't know what the offset was on those 18's that you mounted, but the overall size of the wheel had something to do with it, i'm sure. in my opinion, anything larger that 16" on an integra just doesn't look right anyway.
i think stock offset is +45mm, but i'm not certain.