I use 16:9 and I have started to implement percentages rather than strict resolutions. I still remember someone complaining I didn't have his resolution size... what size is that? 960 x 144. Well then... percentages!
At some point I tried letting people type in the value of their own resolution. It had hilarious results as people would type in resolutions far too small to see or correct it. So then I had to put in a five second fail safe to restore the screen.
Streamers enjoy having windowed mode, so make sure you have windowed mode available and I personally enjoy full screen. Another issue is some people have 2 or 3 monitors and if they want to play on monitor #3 you better let them.
Other than that I've discovered surface stretching where the surface can be stretched to specific resolutions. You need to tweak it a bit so it doesn't look like an ugly mess. I have a few monitors 4:3, 16:10, 16:9 and a TV to use just for screen tests. Now that I have it set, I don't ever want to touch it or mess around with it. It turns into a compatibility nightmare.