If you’re currently jailbroken thanks to Evasi0n you might want to steer clear of that iOS 6.1.3 update as it patches one of the exploits the jailbreak tool uses.
So if you update not only will you lose your jailbreak but as team Evad3rs dev-member David Wang (Planetbeing) says you might need to wait a long time before a new exploit is found and jailbreak is available again.
This story isn’t exactly new as iOS 6.1.2 also patched an exploit the Evas0n jailbreak was relying on but back then a solution was found pretty quickly.
Apple has been issuing updates left and right to fix the passcode lock vulnerability but it seems that each time it does a new way to bypass the lockscreen pass comes around.
So you should ask yourself if the update is really worth it to you and if it’s a higher priority than having your A5-A6 iDevice jailbroken.