Pretty common issue, can't think of anywhere else it would leak from, apart from perished crankcase breather hoses. When I did my rocker gaskets, I also had to put a dab of sealant on the leftmost passenger side bolt nearest the sill as it is also tapped through the head (oil passage). When re-installing covers, torque in sequence and to 4NM - Don't over tighten! Don't forget red Permatex for the half-moon seals too!
Refer here:
http://www.skylinesaustralia.com/forums/topic/292400-rocker-cover-gasket-replacement/