I leave mine plugged in overnight every night - always have with every iPhone and iPad I have had. The phone controls its own battery charging, and simply stops charging the battery once it has reached full capacity.
Partial discharge and charge cycles also will not harm a lithium battery. I too often plug my phone in when in my car, or when in my office at work, when sitting in the airport or train station if waiting and an plug is available. No point in not keeping it topped up if convenient to do, so it is always well charged at the times you do need the battery to last awhile.
Industry standard SMART lithium battery technology is very robust and inherently guards against over-charging. Any quality brand name lithium battery, in any device from an electric toothbrush to a electric car, should be fine if left plugged in after the battery is fully charged. That's the very reason for the industry standards in the first place, so you don't have to worry or baby sit your battery powered devices and their charge state.