In my opinion, Work on Writing is very important and should be launch as soon as possible regardless of the age of your students.

If you launch Work on Words first, students may have the false impression that their spelling is what’s most important when they write. I agree that you should launch Work on Writing asap.

Well, I think the purposes of Work on Writing and Word Work are so different that you might want to rethink that. Remember that writing is a continuum of growth from drawing, scribbling, random letters, phonetic spelliing, etc. So all of those should be accepted in their writing journal. We want them to realize the purpose of writing–to share ideas–so work on writing needs to go next.
Work work is really the “refining” tool for writing, so will follow and help the kiddos move along in their development–but I do really feel building the love for writing should come first .

Thanks again for all the ideas. My students are very concerned about spelling words correctly. Seems like I’m almost having to force them to “underline and go on!” I finally started suggesting that they say the word slowly, write down all the sounds they hear, THEN underline and go on. That has helped some.
It’s rewarding to watch them write - most of them love it, and would probably spend most of the day writing once they get started.
Which leads to another small issue: they are so used to writing “their own way” during Daily 5 that they seem resistant to using any instruction they’ve received - “topic sentences, main ideas & details, etc.” Maybe I’ve stressed the “get your ideas down on paper” too much?

Are you adding often requested words to your word wall? That will help take away some of that “how do you spell . . .” syndrome :). Nice they love to write so much–that will also help them build their own collection of known words.
When you present your writing lessons, are you using mentor texts or books for examples? I find that helps kids understand why we want to try using those “writing styles” in writing during Writer’s Workshop time. Keep this time separate from their “Work on Writing time”, so they note that both are important.