Should I keep my device plugged into my ac adapter

It depends. Some devices sense when the battery is at a 100% charge and stops accepting a charge from the ac adapter. Some devices do not have this capability. For devices that do stop accepting a charge at 100% then it is fine to keep your device plugged into an ac adapter.

More Info:
When charging your battery you are forcing electrical current into a battery cell from a charger. The force of electrical current causes temperature increases. Over time increased temperature levels will diminish the battery's capacity – the ability to charge and power your device. That is a fact and a natural consequence of batteries today as we run through a charge-discharge cycle. In short, the practice of keeping your battery charged continuously can negatively affect your battery’s longevity.