If all you’re doing is opening the JPG in Photoshop and resaving it, no.
If you need to touch up the photo and are going to be doing saves along the way, you’ll want to save it as a TIF first.
Hey Bob, I'm not doing anything to the images in Photoshop. I'm exporting images from Lightroom to use in Indesign. If I need to do any fine editing in Photoshop I'll use the 'edit in' feature in LR so that all the files remain in my LR catalog. It's more whether the quality of a Tiff is noticeably superior when I place them into Indesign that I'm most concerned about. Or will a jpeg exported at the highest quality settings and at 300ppi be sufficent?
You have to consider the amount of compression used when saving the JPEG, and once a file is saved as a JPEG the degradation caused by the compression level is permanent. So if you open a Camera RAW file where there's no compression, save it as a low quality JPEG, close, reopen, and resave as a TIFF, there will be no benefit because the original JPEG compression artifacts were baked in to the TIFF.
And will it make my Indesign project, and any subsequent pdf's I export substantially bigger in filesize?
When you export a PDF the amount of compression is set in the Export dialog (it can be none). The export just takes the pixels that are placed and compresses them. So if you place a JPEG and export with JPEG compression, the placed JPEG pixels will get compressed again. Placing JPEG vs. TIFF wouldn't get you smaller exports. All JPEG will do is save some disk space in your links folder.
Or will a jpeg exported at the highest quality settings and at 300ppi be sufficent?