I am glad you enjoyed the class. When doing custom work I generally ditch as well. A couple of exceptions are when the client doesn’t want any SID and if there are issues such as too much fullness, wavy seam lines, etc.
Threads for SID – I try to match the thread to the ditch side.
No, I do not do all the ditching first. Many recommend this and it works well for them. I generally do the detail/design/fill work first and then do the ditching after. I do it block by block if the thread will be the same. And then the ditching between the body and borders of the quilt last.
Of course, it can change quilt to quilt, but that is then general flow that I use.