Hi Chrisd596,
(We're also from Marlton!)
I'm not sure if this is what you're asking, but here goes-- I booked my reservation on Spirit last February for our trip this past June, so it was more than 90 days out. At the time that I booked, they did not do seat assignments -- seats were assigned by the order in which you checked in at the airport. A few months after that (April, maybe?) Spirit began to accept seat assignment requests. I heard about this and called to see if could request seats (I wanted 5 as close together as possible), but was told that because I had booked online this wasn't an option for me. Now, I don't know if this was because it was something new they were implementing and wasn't retroactive to reservations already made, or if this is not a feature that can be used when booking online. I have a credit due to a delayed flight coming home, and I was told in order to use this I can't book online; I have to call. So some stuff you can do when you book online, and some stuff you can't.
Like I said, I'm not sure if this answers your question, but you can always call the # on the website to check!