Pimskeks, one of the hackers who worked on the iOS 6.1 jailbreak, recommended that users should avoid performing an OTA Software Update to iOS 6.1. Basically hacker says that a restore done through iTunes allows us to jailbreak without any problems, an OTA update may cause random errors during jailbreak. In the absence of other information, we recommend that you restore through iTunes and not OTA Update.
MuscleNerd also posted that straight OTA Update involves a complicated process of verification and testing solution iDevice jailbreak and update sites have made in this way have SHSH other sites, so if you did OTA update, as well to make a clean restore with iTunes.
So if you are planning to jailbreak your iDevice this Sunday, we recommend you to do a backup and clean restore with iTunes. Here are Direct Download links for iOS 6.1 for iTunes restore.