It depends on where you're flying to. I didn't pay for EBCI on my last SW flight, PVD to MCO. I completely forgot to check in at the 24 hr window..missed it by an hour or so. I ended up in the last B slot!
That's the main issue. I always check in exactly at 24 hours.
Issue I have is that SW doesn't guarantee anything with EBCI. They appear to sell as many as people will buy. Your boarding assignment is in the order of EBCI's purchased. If you request EBCI late you can still end up with a high B. They should limit EBCI to A boarding only in my opinion.
At the gate you can purchase A1-A15 seating if seats are available. It used to be a flat $25. It's now $30-$50 based on flight.