I am not fond of building more than the min number of ships, regardless of the type.
The key is what is the min for a given map.
So if you are going to be sending troops via ships and they could be attacked, they need escorts.
Coastal bombardment by Frigates is not a problem. They must get next to shore, so send over some cannons and hit them. If they loose a few HP's they will leave. If you are going to see lots of them, make a ship to finish them off after the cannons do their bit.