: Is it easier to put the rudder on during the build and then not use it or and
: it only if it is needed after some time in the boat?

Depends alot on the design of the boat. A boat with long tapered ends (overhang) will be much more difficult to add a rudder on than one with a more plumb (straight up and down) stern. Either way if the boat has either and end pour or wood added in the stern it's easy to add a rudder afterwards.

Adding a skeg afterwards would be more difficult, though not impossible if it's got hatches.

Bill H.