The company I work for develops iOS apps for children. We currently support all devices going as far back as the 3GS. That being said, our apps would run on the the following resolutions:
480x320 - original iPhone
960x640 - retinal iPhone
1136x640 - iPhone 5 Retina
1024x768 - iPad
2048x1536 - iPad Retina
We're currently using Unity and a on-size-fits-all approach where we use a generic resolution of 1366x768. This produces a few problems:
-This resolution doesn't fit anything exactly so it skews our graphics making our artwork kind sort of fuzzy on everything
-All assets have to be noticeably larger resolution textures on smaller devices (128x128 only takes up 80x80 pixels, for example)
-Massive performance issues due to filtrate bottlenecks
-Memory issues due to larger textures (the 3GS doesn't load)
Just to address that fill rate issue further: our games are small click-and-react types right now and just about every scene uses a padded 1024x1024 background texture and a 1024x1024 texture atlas for our game object. We're drawing maybe 18 - 20 things onscreen at a single time in yet, we still have very choppy frame rates.
What we'd like to do is develop a game with the following criteria in mind:
-Use one universal app
-Keep the game under 50MB
-Try to stay away from DLC to download assets that don't fit into the 50MB, if possible
My approach would be to develop all of our assets based on a generic viewport of 1024x768, which is based off of the original iPad's resolution. Then, just crop for the iPhone's 960x640 display (I'm ignoring the iPhone 5's 1136x640 display for the time being). I'd use a 480x320 display for the 3GS, iPod 4 and iPhone 4 to avoid fill rate catastrophes. I'd generate the low-res textures by rendering the high-res ones in the bundle to a texture at 50% scale, then release the high-res ones from memory. Loading will take much longer on these slower devices, but at least we don't need to keep track of duplicate assets that'll bloat our bundle.
Another approach would be to use a generic viewport of 1136x768 to account for the iPhone 5 and low-res iPad resolutions. Then, continue with the crop-n-scale routine mentioned above.