I've been JB for a few months and haven't updated my phone when it pops up saying there is an update out of fear of messing something up. Are you able to do those updates if you're JB or will it brick my phone?
You are not able to do the updates. It will not brick your iPhone, but if you update you will lose your jailbreak. 6.1.2 is the last version where you could update to that was still jailbreakable. But Apple is not signing 6.1.2 anymore and now, probably not even 6.1.3 since 6.1.4 came out.
When Apple does not sign a version of iOS it means you cannot restore to it or update to it. You can only restore or update to a version Apple signs, which in this case would be 6.1.4 which is not jailbreakable. In the past there have been ways around this, but with the 4S and the 5 those methods don't work anymore.
This is the downside to jailbreaking. You have to wait until there is a jailbreak of the current version before you can update to it. When you do, you have to RE-jailbreak and then restore your data. It's not as simple anymore as doing an OTA update. So, your updates have to be planned and they have to be done within the window of time that Apple is signing updates and that the jailbreak keeps up with those updates.
That time for iOS 6.x is over. The next jailbreak will be for iOS 7. So, if you want to keep your jailbreak you are on your current version until iOS 7 has been jailbroken. That could be a while.
Depends on the current iOS version an which JB you used.
6.0-6.1.2 with evasi0n will not let you do an OTA update on the phone itself, you can only update through itunes. If you do that it will update the iOS and you will loose your JB.
Depending on which device you might or might not be able to downgrade again to previous iOS version.
I've been JB for a few months and haven't updated my phone when it pops up saying there is an update out of fear of messing something up. Are you able to do those updates if you're JB or will it brick my phone?
Software Update Killer is that tweak in case you're curious. Also install ResetAllKiller too while you're at it so you or someone else doesn't accidentally press the erase all content and reset settings button (which will mess up your device and jailbreak)