There's a setting for that https://yabb.jriver.com/interact/index.php?topic=100055.msg693178#msg693178
Flight is right, however if you are really paranoid about it (just in case this for some bizarre reasons flips back to a default when changing versions) go Options=>Library&Folders. In the filterable field list you can uncheck the 10-15 fields that are really important that Picard is handling and uncheck "Save in filetags if possible" (being aware of the mapping names like LABEL=PUBLISHER etc.)
One thing to note if I may, JRiver is not like iTunes ... the changes it could make to files would be statistical (like number of plays), or administrative. Basically the option to not write to files and/or unchecking the ability to do so by field is to protect your metadata from
your "accidental" changes. It won't change your metadata unless you tell it to, or unless you have set it up to auto look-up metadata through their database. (at least for audio ... not sure about sidecar files, but as they are not embedded ...)
Personally I re-tag a lot via JRiver and when needed I call external programs from within the program (right click => Send to). I always set to "on" the update tag options. However I protect certain fields by unchecking this box -- which may also be an option for you to consider.