Production Meetings Are important to keep the Designers focused on their task and also in the loop of all the crazy antics that are going on in rehearsal. Production Meetings serve as a relay between the Director and the Designers and to make sure that they stay on track. These meetings are to discuss the progress of the play, any large and difficult technical issues, the rehearsal schedule, and any other issues related to the show
The Director, the Stage Manager, Producers, and Designers are the ones who attend the Production Meeting. While it may not be possible for all those individuals to attend, not having them there is only to your detriment as they will miss important details. It is hard to find a time for so many individuals to meet, but having them all there will help more in the long run.
The amount of Production Meeting in a show's lifecycle vary and your shows needs maybe different from other show's needs. Typically there is one at the beginning once you have all your designers and your stage manager before you have auditions. The next one should probably happen after your first read through, it is likely all the designers will be there, and now that you have a cast it may help finalize a few things. Your last one should usually happen after your Designer Run through, this is your most important one as it will be the last chance to finalize tech elements before you go into Tech Week. This is the Designers last chance to ask questions and decide on looks before it has to be set in stone.