Hello Richard,
You don't need xmp sidecars with jPGs , and you should not use them, JPG is a perfect container for xmp.
Concerning NEF, you should follow John's advice, IMatch is really geared toward using xmp sidecars with Raw and if you use Adobe programs it is definitely the way to go, they will anyway write sidecars and your metadata will be out of synchrony between sidecar and Nef unless you are very careful and organized.
This said it is perfectly possible to use NEF, Imatch and no xmp sidecars. I do it and it works fine. But I don't use LR, PS etc... What I do:
- in Preferences/Metadata2 keep everything by default apart from XMP Export/Allow to write XMP files that I set to NO
- in the same panel in Configure Fule Formats, for NEF I have "XMP sidecar Files" set to "Embed XMP in File"
- I also have Write IPTC, Exif and Allow create IPTC/Exif/GPS all set to yes, but this really depends on how you want to handle your metedata.
If you want to try, do it on a test folder until you are convinced it works like you want. A good way to verify what is written or erased from the NEF is to use ExiftoolGUI.
I do without sidecars for "historical" reasons, always used programs like IM3 that allowed writing metadata to the NEF, so I have tens of thousands of these with embedded xmp. If I used LR like many here I would have exported my metadata from the NEF to sidecars...
Francis