If the T is aware you have DID, I'd say whenever you feel is appropriate is good. In my own system, there's actual danger is we don't switch fairly early because otherwise the outside persons are established as "people we have to be John with" and there's tremendous internal pressure not to present as anything other than John. This is left over from childhood and even though we know it's no longer applicable, there seems to be an internal mechanism that stands in the way.
I also think early is good because you get to see how the T responds to the alters. If the T has trouble connecting or treating them as unique individuals, or doesn't give them a fair shake, or is uncomfortable and wants the host to come back, it's better to know that sooner rather than later so that you can
find a new T! My alters pushed for the verdict expressed in the previous sentence. My T has been extremely welcoming and responds to them sort of as if they're just a part of me and as if they're a new person who stepped into the room. It's an impressive balancing act and she's very experienced with DID, but they're still a bit wary.
I read it in a book that one of the alters, or the host, needs to be responsible for going to the meetings. The host doesn't like to talk, and won't bring up serious issues, will lie about how she's doing and flat-out sabatoge the session. So I'm thinking I, or Jo, should take over the sessions. Maybe Jo, because she holds more memories than I do. She shares them with a couple of the others, and knows more about the others than me.
We've decided that it absolutely must be more than just me attending. I try to be as honest as possible but I'm just don't see everything and I can get overwhelmed at times by the damage that was done to me but not to the others. Plus the other are absolutely parts of the whole mind. It can't be just me or we'll probably be in big trouble. Based on our experience thus far, however, it appears that EMDR may need to be done by me. That doesn't really matter because whoever was experiencing the trauma at the time originally tends to show up anyway, but not so much as them or as me, just as the child we were when it happened.