Definately PREbook your seating arrangements. I just had a travel nightmare, after reading and talking with several people, they said it did not matter if you prebooked your seats since Delta doesn't use them anyway, so we didn't. Actually, when the screen came up to book the seat, they didn't have five together, so I didn't bother anyway.
Well, when we went to check in, we had the hardest time getting any seats, since we were not PREbooked. We actually had to wait on all three flights until everyone boarded and THEN they could see what seats were open. Kinda crazy, since it is all in the computer anyway. Then to top it off, they couldn't find more than two seats together. One flight, they had a very hard time getting two seats together which I have to have due to a child travelling.
So, any way you can do it, Do make sure you prebook your seats.