For everyday travel bookings, I don't bother with a TA either. But for something like what you are planning I would be consulting a TA along with my own research. It's not like you pay the TA. But its your holiday you can do it however you want. Why would a TA give you any heat for you spending your own time doing the research and booking it yourself? If it all goes wrong, its not going to be their fault.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013