If you are on the Magic or Wonder, there is almost no chance of having to wait. As above, the issue is to spread out the crowds and shorten the lines as well as to not have more people in the terminal than the fire marshall will allow. If you are on the Dream and arrive before boarding starts (11:45) with a later time, you may need to wait a short while. As soon as boarding starts, there is no longer an issue with the terminal being too full.
If you arrive after your time, they will just let you in; they are well aware of issues with late flights, etc. It is not a problem....well, as long as you are there before all aboard time!
If you arrive by
DCL bus, no problem although they have held people on the bus for 10-15 minutes at least a few times (I suspect waiting for crowds to thin).
ou can check in whenever you get to the front of the line; the issue is getting into the terminal. People are rarely kept waiting more than a few minutes. However, checking in early will not get you a better boarding number. That was assigned by the computer based on your scheduled port arrival time.