It's all about keys. If you need a key to open the door, make sure you'll have a key when needed. You need several of each key, aside from the one on your keychain. Your life is worth spending the extra couple bucks on keys.
Here's where the keys fit, from most used to least used:
Keyring key: This stays on your keyring like every other key you've ever used. It's your day to day egress key, often used in conjunction with a car.
Convenience key: Keep this key in the door most of the time, except when gone. This works best with an entry alarm you can arm at night, if you'd like to keep the key in overnight, which isn't a bad idea unless you have extra security needs than normal. A hook on a wall a few feet from the door can hold this key.
Aux key: keep another key on a hook by the door. Not right next to it, where it can be reached/fished from the window, but close to it. This is for guests and absent minded residents to lock/unlock the door when nobody else is around and the convenience key is out. It's also the key you give to house-sitters when you go on vacation.
Think of one belonging to the door (a knob), and one being a floater. You can combine the above two if you don't have a lot of company/family. The main point of two non-keyring keys is to avoid practical situations in which you're w/o a handy key, or tempted to touch the emergency key.
Emergency key: Keep a key under the inside doormat. You can also use a Command(tm) hook or nail halfway from floor to knob to hold such a key, so long as it's available from a crawl. This is for fire/emergency use only, don't ever lend it out or misplace it, and remind any overnight guests where it's located. A red key or tag can help avoid mix ups; you DON'T want to be surprised that someone never gave it back...