Honestly, everyone wants more precision (like 1% at a time), but the real problem is accuracy anyway. I'm not sure how it rounds, but I can tell you pretty much that the baseline info is not that accurate. I spent a few hours the other day draining my battery completely testing a Metamorph theme I was making with Smiles for battery levels. It took me around 3 hours with screen on full brightness playing podcasts and running maps/gps to get to 0. The thing is with the same load it took a lot more time to drop from some levels than others. The last 5% oddly, took longer than any of the 10% increments before it (yeah, like that'd happen when I didn't want the phone to die). My point is the load was the same, but the ability of the OS to predict what % is remaining is surely less than 5% accurate, so you're not likely to be better informed about your true battery life remaining by knowing which way it rounds.
Incidentally, the second drain with a fully charged battery took 20 minutes less, and yeah, it was at 100% on the wall plug for over an hour before I did it a second time. Judging battery remaining is just more guess/art than science, sadly.
