Yesterday, Apple released iOS 6.1.3 unto the public, and it came with a couple of big updates as well as some general bugfixes and performance improvements. However, it also patched up some of the exploits used by the Evad3rs dev team in order to jailbreak iOS 6.1, which means that if you update to iOS 6.1.3, you'll lose your jailbreak.
Of course, some iOS 6.1.3 users can still jailbreak thanks to Redsn0w, but it's limited to devices running Apple's older A4 chip and it's a tethered jailbreak, meaning that you have to plug in your iDevice to your computer every time you reboot it, which can be a pain in the rear end most of the time.
However, if you're currently on iOS 6.1.2 and are either jailbroken or have plans to jailbreak in the future, don't update to iOS 6.1.3 -- at least for now. Evasi0n team member David Wang (a.k.a. Planetbeing) says that he and the rest of the dev team don't yet have a fix for the iOS 6.1.3 update, and they aren't sure if there will ever be one.
Of course, you may be thinking about that pesky lock screen vulnerability that's present in iOS 6.1.2, and while I do care about security, there's probably no reason to be worried about it. If someone got a hold of your iPhone, the chances of them knowing that elaborate scheme of swiping and tapping gestures to get past the lock screen are low. Of course, there's always that chance, but you'd probably have a greater chance of getting struck by lightning if anything. Oh, and another lock screen vulnerability has been discovered in iOS 6.1.3, so you're screwed either way.
Plus, iOS 6.1.3 doesn't consist of a lot of new features anyway. Besides the lock screen fix, there are improvements to Apple Maps specifically for Japan. When's the last time you were in Japan? Have you even been to Japan? Probably not, so it's safe to say that you don't need the Japan maps improvements, or any of the other new things in iOS 6.1.3 for that matter. (Japan residents need not apply to this.)
In the end, it's best to play it safe and stay away from new updates if you're jailbroken or have plans to jailbreak. Heck, I'm still running iOS 5.1.1 and I'm chugging along just fine like the rest of the iOS crowd. We may eventually see an untethered jailbreak for iOS 6.1.3, but it could be awhile, and if history is any indication, it may be even more difficult to jailbreak iOS 7 than any other version in the past.