Correct, that is the way it works. You can call beforehand to check on available seats. However there is the possibility that the time between you talking to a Res agent and clicking on Purchase, someone else could theoretically snatch up the seats you were looking for.

There are a few seats held back for assignment day of departure at the airport. You could grab seats then although it requires getting to the aiport well in advance of your flight to make sure you get them and not someone else.

Supposedly seat maps will be available before purchase but its apparently behind schedule and at this point no one seems to know when it is expected to be available.

I would say if less expensive fares are available and its far enough out from departure, there's a very good chance of getting seats together.