16 x 20 actually works very well for competitions or displays that include multiple prints from multiple photographers - the sort where prints are expected to be mounted but not framed.
The prints almost seem to be straining to jump out of the mat, which can be positive in the right circumstances.
For framing, I'd probably lean toward 18 x 22 or 20 x 24.
Thanks for the useful suggestion. I also noticed that a 12x16 mounted on 16x20 looks just about fine without a window overmat, but seems strained when overmatted and framed. So while convenient, matting 12x16 to 16x20 makes it hard to frame it later, meaning that a reprint would be necessary, hmm... Ideally, I'd settle on one print and mat size, and deciding that gets even harder when I think of square prints.