The time is set for 6pm CST; however, if you have another obligation that will prevent you from opening at that time I do not have an issue if you open early. However, that does not mean we all open early. Part of the fun is opening as a group and sharing the photos.
Be sure to PM your partner with a thank you. Even though you may say thank you on the board it is also nice to receive that PM of thanks.
Also remember there is nothing wrong with giving your partner a rating. Ratings do help other hostesses when looking at people who want to join their swap.