With the release of Greenpois0n RC6, people have been asking whether or not they should re-Jailbreak their devices. Well, it is up to you.
I've had the RC5 B4 Jailbreak on my device with no problems at all. Some people have been having problems with emulators or just want to Jailbreak their Apple TV 2g, that is the reason for RC6. It is just a bit more stable, more things function correctly, but like I said, RC5 gave me no problems.
It is your choice if you want to re-Jailbreak or "Double Jailbreak" a.k.a., Jailbreak over your old Jailbreak. You wouldn't lose any data, but it is always good to back up. Hope you all didn't have any problems, and happy Jailbreaking!