I'm a proponent of doing everything but the seat stays together. It's really easy to get movement between parts if the joints are done one at a time. If you have round sockets for the CS, then you might be giving up some freedom by doing the ST independent of the CS. Oval CS seem to have a lot more freedom
But the traditional way of doing a piece-by-piece build is to braze ST to BB, then HT to DT, then put everything together. I have done this a couple of times and it worked fine. I think if the DT and ST were already in the BB, I would just finish tacking all the front triangle joints now