A lot of us who are planning to buy the LG Nexus 4 will be doing so because of the developer options, including the easily unlockable bootloader. Unlocking your bootloader is something to think about before you jump in and do it, because a locked bootloader is essential to keeping your device secure. Once you unlock it, a knowledgeable person can take every bit of data off your phone, even if you're unrooted and have adb debugging turned off, and even with a secure lock screen.
Now that's out of the way. (Seriously -- think before you take the plunge, OK?) If you're going to want to unlock your bootloader, you'll want to do it before you have the phone all set up, because unlocking it erases everything from your phone. That's a security feature. If a hacker gets your phone and it is not unlocked, he can't get into your data to steal it as easily. Security is good. So make your decision before you dive into the Google Play store and set things up just so.
With all things considered, if you're going to unlock it, hit the break and see how to do it.