In the early game unless I need the ships for transport (Sicily, Venice, Portugal, Byz etc) I do not build any. If I am blockaded, I build enough ships in that and nearby ports to break the blockade, but I do not keep them waiting around because the upkeep will drain you so much for the tens of turns that you are NOT blockaded, that your ports might as well be blockaded. Once I have a good economy going i will keep a few around because the upkeep is not that much compared to total income.

People tend to severely underestimate the effect of upkeep on their economy. For instance playing as HRE you will soon be in debt unless you go rebel blitzing. But if you simply disband 12 units of peasant archers you start with (1200 upkeep), you will start the game making a profit, and no one is likely to attack you in most of your provinces right away.