Using an unlocked bootloader on your phone or tablet is not safe. Don't do it. Unless you want root permissions, or the ability to backup and restore your software via a custom recovery, or you want to use a ROM that didn't come with your machine. These things don't make an unlocked Android device any safer, but they are pretty good justifications for unlocking your bootloader. If you'd like to have all that unlocked cake and metaphorically eat it too, the BootUnlocker app is your friend.
If you like to mod your Nexus devices but you're also a fan of tight security, you probably already know BootUnlocker. It's a simple app that allows rooted devices to lock and unlock the bootloader without wiping user data. The developer, segv11, is back with v1.5.1 of this handy little utility. The latest update adds support for the WiFi (flo) and LTE (deb) variants of the 2013 Nexus 7 and the ability to set the tamper flag on the Nexus 4 (mako) and Nexus 5 (hammerhead).