Good questions.
I will answer on my end, and I think Mands would agree. With all clients him and I talk about their current state, experience, goals etc. We put our heads together and establish a plan. That includes diet, training, supplements etc. Of course at times we have different ideas, and if they were big issues, we would consult the client with the two options and the pros and cons of both. However, with most things, Mands and I are on the same page.
We have written much more advanced programs for more advanced clients, and very simple programs for newer hobbyists. We have no desire to push someone past their comfort level.
I think it can never be too early to use a coach. I wish I did way earlier. The things I learned working with other people are truly invaluable.