I certainly know what everyone's talking about with the dreaded 319-->320 swap (if you're in 12A/F), and the dreaded 320-->319 swap if you're in row 3 of FC.

My experience, however, has been that 9 times out of 10, when the computer says it's a 320 but it ought to be a 319, it actually is a 319 and the seatmap is correct for the 319. I've even had times where the gate agents thought it was a 320 when they looked at their computer, but it was in fact a 319. Also, I could be wrong, but my sense is that those swaps happen more often on shorter notice at the airport, rather than as the result of any day-before or day-of change that appears in the computer.

Bottom line: I no longer sweat it when my 319 says "320" on the computer.