EPS is not a lossy format. But the format you use for your raster portions can be (the JPGs you are using are always lossy, and you lose something with each successive save).
In either case, you should not be using JPGs for print production. Stick with TIF for raster to ensure you don't introduce unwanted compression artifacts. TIF does not have to be lossy to save on space, you just won't get the same space savings as you would with JPG. I would argue that those benefits are negligible at most. Just go with TIF for print.
Unless you are applying text to an image in Photoshop, exporting to EPS is completely unnecessary, and I would recommend going with TIF. EPS adds a wrapper of code that isn't required here.
If you are adding text to the image, I would suggest placing the image into an Illustrator document, adding the text there, and then saving that as an EPS. There are clear benefits to doing this: you get to keep editing control over the text and you get all the benefits of vector (extra sharp text for one).
UPDATE based on OP's edit:
A JPG is not "already perfect". JPG is a lossy format that the image needs to be converted out of, even the lowest compression setting is still compressing and leaving an artifact somewhere. But EPS isn't the correct one even you are adding text to the image later. For examples of how the compression works and just how bad it can get please see the following:
You are right that a single edit to a high resolution file may not make a practical difference. But what happens if the application you are working on saves the JPG in a different, higher compression than the original? Those artifacts will compound over time.
You need to convert your art out of JPG and into something more appropriate format like TIF. It's not hard, and setting up a batch conversion in Photoshop is easy.
Just because the stock photo companies store their content in JPG does not mean you can actually use it in that format.
Update to answer the questions directly, per the OP's comment upon acceptance:
...is it necessary to save it as the
EPS in the step between Photoshop and
If you are not adding text to the image when editing it in Photoshop, then no, it is not necessary to save the image as an EPS. If you are adding text, then that work should be done in Illustrator and saved out as an EPS.
Am I missing something other than
lossy/non-lossy about an EPS that
makes it better for this?
Yes. I think you are misunderstanding the roles the various file formats play in print production.
JPG was created in 1986 to offer a means to store a lot of image data in a smaller space; JPG was created in a time when disk space was small and very, very expensive per KB (note not MB because that was a very rare metric in the late '80s). JPG was adopted by the web because it offered distribution of reasonable quality art and photos with minimal data volume, which is just as important today as it was in the 80s and 90s, and is part of the reason why the stock agencies still use it for distribution over the web. JPG was never intended, however, to be used in high quality print workflows and for multiple iterations of edits on the same file specifically because of the lossy compression that comes with the JPG standard.
EPS is a subset of the full Postscript language, and is primarily geared towards vector art. Raster art (i.e., photos) is non-native to EPS, and subject to all sorts of glue code to include it. Since Postscript is a "fat" language, inclusion of art can be done in several different ways. Since Postscript is holding the art in its original form, it is not guaranteed to be non-lossy on lossy formats since the content can be, and usually is, refactored at one point or another during a print workflow. It does make sense to import raster art into a an EPS when there is text or important line art is to be added (i.e., labels and leader lines).
TIFF was created for variety of reasons, but since it is non-lossy by default—a TIF can still utilize JPG compression but that needs to be a deliberate (and foolish) action on the part of someone—it became the de facto format for graphic designers since multiple edits didn't result is an actual loss of integrity of the core image data.
...if the original JPEG is otherwise
already perfect is it necessary to
convert to a non-lossy format first?
From a purely technical standpoint, you could just use the JPG if you edit once. But as you can clearly see, there are many historical reasons why I extended my answer to state that is not in any way, shape, or form, best practice.
No one has any way to accurately predict how or when a piece of art is going to be used. EPS is overkill here just by the jump in data size alone and still not a guarantee that the integrity of the image will be maintained throughout the project's lifecycle and by extension for the life of the art. Best practice clearly dictates that you convert your JPGs to TIFs before doing anything with them and use the TIFs instead.