Laura,
The ladies are doing a great job of talking through this common question!

As far as CM goes, she did not encourage original writing in the younger years, as she wanted the children to develop the habit of seeing words spelled correctly (mainly through copywork).
In my 11 years of public school teaching days, the new philosophy was getting kiddos to write "as much as possible and all of the time... and don't worry about the spelling". Inventive spelling was "in" and children wrote volumes of incorrectly spelled work, which I could never get on board with!

As the years passed, teachers were discovering exactly what CM found... that the habit of seeing words spelled incorrectly and writing them incorrectly is a difficult one to overcome. While some kiddos are naturally good spellers, the rest of the kiddos were developing terrible spelling habits and having no sense at all of what "looked right" anymore. So, when I headed into homeschooling and read more of CM's philosophy, it really made sense to me in this area in particular.
With my own kiddos I do not discourage them from writing original stories and sentences, and they all do it on their own during free time. Often great literature inspires them to be creative writers on their own. I don't correct their "free-time" writing and just try to compliment on the content. If there is a glaring spelling error I may point that out to be fixed.
During school-time, we keep their early year's writing practice to copywork, and then in grade 4 on up begin moving onto some original composition as planned in the HOD guides. I do require correct spelling for school related work and will often write their ideas on a markerboard to be copied correctly. I do check every paper they do during school time for spelling and require correction. I write the correct spelling in pencil above the incorrect word (or in the margin) and have the kiddos erase and copy my spelling, then erase my word, so only theirs remains.
I realize their are many ways to approach this issue and that the CM-style of teaching spelling is just one way. But to me, it has made the most sense of any that I've tried.
Blessings,
Carrie