1995 Pontiac Grand Prix weeping head gasket

Hello,
My 1995 GP has a steady coolant loss, about one cup a month. The intake manifold gaskets are new, replaced a few months ago. Recently, I found a sludge on the passenger side of the rear head, where the head meets the block. I showed it to mechanic, and he says the leak is not serious and I could continue driving the car. Of course, he can replace the gaskets, but the repair is costly and he says these vehicles could be driven for years with weeping head gasket. Do I need to replace the gaskets right now anyway?