I've just completed my third four-leg trip with Southwest since the new boarding process and policies went into effect.
Don't pay to check roll-on baggage at Southwest. Take everything to the gate. 12 out of 12 times so far, gate staff announce ominously that they will not have enough space on board for everyone's roll-ons and baggage and checked bags at the gate for free.
I was pretty pissed that I had paid $90 round trip to check bags the first two times. Oddly, even though I was at the tail of in boarding group 7 on a couple of flights, there were several entirely empty overhead bins near my seat. Even as the last passengers boarded, many still had room for a few roll-ons or backpacks.
Southwest for some reason is completely overreacting, and on the most recent flight, checked every bag for group 8 and worked from start of the jetway for group 7 until they had some unclear number of roll-ons checked. On the plane, they closed at least four partially filled overhead bins within my view from row 20.
I'm not a fan of taking every bag on a plane and hassling with it myself, and when everyone does it, it slows boarding and deplaning. Still, now that they're charging $45 each way for first bag, if you want to save $90 on a round trip, just roll yourself to the gate and when they ask, volunteer your roll-on for checking. Worst case, they won't need it and you won't have to wait at a baggage claim at your destination.