We prefer the late seating. We like it because you don't have to hurry to get dressed for dinner when you come back from touring around. The one drawback is by the time you get finished with dinner you may miss the main show, or possibly not got the premium seat you would like.
My TA did request 6:15pm for me, but I don't know if it will make a difference. We have had 5:45pm on our last 2 cruises and are hoping we get the 6:15pm this time. You will find out when you check in for your cruise. If it is not the dining time you prefer, you can ask the maitre d (sp) to switch you (there will be a time/place to do this, posted on the carnival capers - ship's newspaper). However, I tried that twice and was not able to switch either time, but that could be because we were in a group of 6 - 8 people.