We were on the PG 2/'02 and two weeks in 2/'03. Both times early in the month. Out of these three total weeks, we had "rain outs" for two half-days.
That being said, however, when we were onboard both years, we heard stories from the staff about how much rain had been experienced on sailings just a week or two before.
From our experience (limited to that described above), we would avoid Dec. and Jan. if at all possible. That's the worst of the rainy season, as I understand it.