I'm pretty sure there's dedicated hardware to handle video decode and scaling and the difference in battery drain between playing SD and HD is likely minimal. You'd do better extending the battery life by lowering brightness. As for waste of space/bandwidth, bitrates dictate that, not resolution. A 1280x720 video at 3Mbps will consume roughly the same space as a 1136x640 video at 3Mbps. Sure, you might get a bit better PQ with the latter but I doubt it's significant enough to bother with creating 2 separate encodes. The difference from 1136x640 to 1280x720 and 1704x960 to 1920x1080 is pretty small anyway (~12.5%).
There's nothing stopping you from tailoring an encode for each of your devices but there's also nothing wrong with sticking to standard resolutions (e.g. 1280x720, 1920x1080) for your encodes. Scaling can be done by the mobile device on the fly and you're not using the CPU to do the decoding and scaling anyway. Besides, even Android devices can vary on resolution/aspect ratio. I've used devices that are 16:9 (1280x720, 1920x1080) and 16:10 (1280x800, 1920x1200). If your Android version and player supports immersion, you actually get a more distorted picture with a 16:10 Android device when you do stretch to fullscreen than on an iPhone 5/5s (and 6 with rumored 1704x960 resolution).
Unless you're OCD and must absolutely have 1:1 pixel mapping, the not quite 720p or 1080p display isn't much of an issue. Now the iPad with its 4:3 aspect ratio, yep that one's not so ideal for playing 16:9 HD video much less 2.35:1. Way too large letterboxing.
The pic and u/n were taken from separate anime. The u/n is in reference to Hanazawa Rui, a character from the shoujo manga/anime Hana Yori Dango/Boys Over Flowers. If you're somewhat familiar with the Japanese language, you'll know that it roughly translates to Rui's girl. I started using the u/n while I was in high school and I just can't be bothered changing it.
The avatar is Tsukiyono Omi - florist by day, assassin by night from the anime Weiß Kreuz. 
