Once you have your reservation, you can book the shuttle. If you want a specific time, then I would go ahead and book it now.
As to when...
That has been a subject of much discussion, and I think all sides have valid points. It really is your decision. Some will book the 10:00 am shuttle (that would be us), as we like to get to the port, check in, meet friends, take pictures, sign kids up for the lab, etc. The boarding of the ship starts about noon (although your actual time will vary). Some people see this as having to wait two hours, other see it as the first two hours of vacation. Once boarded, you can go immediately to check Palo and other reservations, have lunch, explore the ship, and maybe go swimming.
Others like to arrive after noon, so the crowds are gone, no line to check in, then walk right on the ship. By this time your room may be available, so you might even be able to go directly to your room. You still have time to explore the ship a bit and maybe get lunch before the Mandatory Life Boat Drill.
So it really is up to you. But most important, if you don't get the exact time you are looking for, don't worry, you are on vacation.
Regarding the return trip, there are queues outside the terminal where all the Radisson people go to wait. The shuttle will pick you up there. There are no reservation times.
