It just depends on your circumstances and priorities. I live on the west coast, so the flights I take are almost never connecting flights. So when DW and I travel, we rarely pay for EB unless we know we can't get to a computer at precisely 24 hours ahead. OTOH, we're travelling to Chicago during the height of the Christmas rush this year. I paid for EB for both of us, round trip, months ago, because I know what a hassle things are going to be around that time of year.
Usually, checking in online results in a boarding pass around A30 to A50 for us. But I've also had the experience of paying for EB and getting a B boarding pass. As I said, it depends on circumstances. But I rarely pay for it; and I wish that fewer people would. As more and more people pay for it, it may become necessary to also do so, just to keep up. I suspect that Southwest is hoping that it will get to that point. I hope not.
-Bob