I wouldn't go as low as you go, especially not if you do commercial printing, stuff that people buy. Why not? They're bound to take a closer look at what they've got and may not be pleased if the spot blocks, instead of details. 16x20 final print I would bump up to 2800x3500. Not a huge step up, but enough to even let the image look nice when you look at it close up. The last thing you want is a customer returning an artwork because of not high enough quality prints. I've dealt with enough clients to know that they take a look close up, even when their image is 16x20 or larger. When I render sizes between 8x11.7 inch and (I'm in Europe, slightly different paper sizes than yours) and 16x24 inch, I stick to 2500x3500 for each format between that range. If I need larger, my render size goes up, If I need smaller my render size goes down. At 8x11.7 inch clients tend to be very critical about the quality, so I want it to be good for lots of details. The get a bit less critical when you go larger, but still want crisp details. At least that's my experience...... contrary to what you say, most of my buyers do NOT stay an arms length from their prints. The pick them up and inspect them up close and want the best.

Don't compromise on this too much. Yes, for sure, you don't need those massive render sizes for larger formats, you wouldn't see much difference. Neither can you go too low, that will be noticeable.

By the way, even so all of this is true, once you go commercial, a lot of commercial printers will reject work that's below 300 dpi. I dealt with a client last week who insisted on 600 dpi. Granted, I didn't output my work at the huge pixel sizes that come with that, I simply used software to upscale my 300 dpi image to 600 dpi and he was happy with it. If your dealing with a commercial printer or agent you may want to check what they want. You can still render at less resolution then what they demand and use something like PhotoZoom to enlarge the image to their demands. Not Photoshop or so, they've got terrible upscale algorithms, dedicated software does the job a lot better.

Artwork and 3DToons items, create the perfect place for you toon and other figures!

http://www.renderosity.com/mod/bcs/index.php?vendor=23722

Due to the childish TOS changes, I'm not allowed to link to my other products outside of Rendo anymore :(

Food for thought.....
https://www.youtube.com/watch?v=pYZw0dfLmLk