Releasing models makes it no more or less likely for fraudulent submitters to get the item in game, Valve asks for the original source files for submissions as well as compiled which no who steals the items will have unless the original author releases the source files too. Money being handed over for an item to the someone other than the author would never happen.
It's for these kind of reason that Valve wont accept audio submissions as authentic proof that the it's the submitter own work is pretty impossible
Actually, yes it does. I have submitted 10 items to the workshop and haven't released anything but pictures publicly.
If I released those models to public use before I submitted, someone decompiled and submitted them via workshop, Valve accepted them, and released them there would probably already be a check in the mail to the submitter.
Then I'd find out when I saw them in game, would complain to Valve... then they'd stop payment... in the meantime someone else could have already received some payment.
So someone can take that picture and make a new 'submission', but it won't have the model files included. So Valve knows it's not the original.
That's my point, if you don't release to anyone except the workshop then a submission of your content is only an annoyance/votes issue.
Like said, it sucks because it stops you from releasing content 'in good faith' to the community to use. But if you release it in a thread here, then it is dated and pretty much verified. This site is known by Valve, so it's about as good as you can get for authintification(sic). You can also post pics of it on other sites like SPUF to 'postmark' it.
So someone else would have a hard time 'pre-posting' to prove it was theirs first.
Releasing at a site like FPS Banana would probably be the worst option.