I think it probably varies from child to child. Sure, there are probably various child development "experts" who will say one thing or the other, but ultimately, who knows.
In my experience, my 6.5 year old son was very interested in forming letters, and loved dictating something to me for him to copy down. This was probably when he was around 4 1/2 years old or so. He loved to write notes to his dad and grandparents that way. He was also much better at sounding out how to spell a word than looking at a printed word and sounding it out. Eventually all the pieces came together beautifully, and now he's reading at a much more advanced level, but his writing and spelling is still probably right at age-level appropriateness.
Really, I'd just suggest that you try to follow your DS's lead, and if he's enthusiastic about writing and highly motivated to do it, it certainly can't hurt. If he'd rather read or be read to, I'd go that route. My guess is that every child will have their own perfect balance that will work best for them.