There is no advantage to waiting until the battery is below a certain percentage (and there may be some disadvantage). Charge it when it is convenient.
Also see the following from http://www.apple.com/batteries/why-lithium-ion/
Charge your Apple lithium-ion battery whenever you want. There’s no need to let it discharge 100% before recharging.
In addition to FoxFifth's excellent answer, I would make an other suggestion. Put your phone on charge every night before you go to bed. Make sure that you have iCloud Back up enabled in Settings>iCloud, that it's connected to WiFi and that the screen is locked. This way, you'll have a fully charged battery every morning and, more important, your phone will be backed up. That way, should something untoward happen to your phone, you won't have lost more than a day's worth of data.
You've gotten 2 good answers. Here's more detail: When to charge your iPhone or iPad
When is the best time to charge the iPhone battery
