It's too late when it's grinding, as this indicates that your bearing races are pitted.
If you don't know when it was last serviced, you might as well service it, since the grease does dry up over time, and despite feeling 'smooth' the grease can be completely contaminated or missing entirely.

I've encountered several pedals, hubs and headsets where it was claimed to be 'smooth' when in fact it was not under closer inspection.

It's not too hard to service the bearings yourself. The tools would be something like $30.
http://www.parktool.com/repair/readhowto.asp?id=127
http://www.parktool.com/repair/readhowto.asp?id=105