So how does Miss Snark feel about writers using editors? Is it worth doing - assuming said writer can find a reputable editor? And then, should it be mentioned at any stage when querying/sending partials, or kept under the writer's hat/beret/green eye shade?
I think it can be valuable if you're not really a writer and need some help getting your idea polished and ready. By "not really a writer" I mean people (and I have several clients in this category) who have interesting narratives (principally memoir) but are butchers, bakers and candlestick makers by trade.
For a novelist, a critique group can perform much the same function.
I'd leave it out of my query letter for these reasons:
1. it's not essential information and you've only got about 250 words so why waste them;
2. it's not persuasive information- I don't give "edited" pages a closer look than any other kind of page.
3. it's not "withholding" information if you don't mention it.