I think you'll have to tell us who your carrier is in order to answer the first question. It's pretty normal for manufacturers and carriers to say that modifying the system software will void the warranty, but how strongly they enforce that varies: some will use it as an excuse not to pay for repairs, others will take a reasonable approach that many hardware faults are clearly unrelated to rooting. Of course some also make it very hard to root it in the first place.
Not draining the battery fully if you can avoid it is the top tip for battery longevity. So what you are doing is fine, and you can top up earlier than that if you want. Heat is generally the enemy of lithium batteries, so using it heavily while charging it at the same time is probably a bad idea too. For what it's worth, I've never done anything fancier than avoiding those two things, and my last phone's battery life didn't significantly degrade until it was about 3.5 years old (people who tell you that any battery will be shot after 18-24 months are people who abuse batteries).
If you have a battery life problem battery monitor apps may provide more information than the system menu, which may help you identify rogue apps. "Battery savers" I'm generally wary of: this is often just another name for a "task killer" ("RAM booster" is another, and one that shows a striking ignorance of how Linux memory management works). Task killers will tell you that they save power, improve performance, etc, but actually they do the exact opposite: even if they don't just end up in a fight with the OS, constantly killing apps that are then restarted automatically, they actually waste power and slow the system because it takes more energy to reload an app after it's been killed than to swap to an app that's in the background/dormant in RAM, as well as interfering with the system's own optimisation. So my advice there would be to not worry unless you have a problem, and if you do find a problem see whether you can identify it using the system menu (or a more sophisticated monitor if you feel the need).
