Depends really. As Shimazu, you're close to 4 different trade ports from the begining. I'm playing on Hard, and pretty much rule the seas as eastern/northern as my provinces go. (In fact the income you get from them makes the game significantly easier)^
Nope, the AI seems to be pretty ruthless with its naval fleets on hard, so trying to upkeep one stack (let alone any more than one) just seems like a waste of income. Not only do you have to upkeep the fleet, but also the trade ships and I think the money is just better spent on upgrading all farms/sake dens/markets etc. and then making sure you have the best combination of buildings on each province for recruiting high stat troops.
If you play as Date on the other hand, there's no way you'll be able to afford a fleet to steamroll all the way down to the south/west to get that trade.