In my case, I always keep the original JPG. But this is because I often lack the time to PP hundreds of DNG pictures. You know, PP the good shots and stick with the JPG of the "so so" shots.
You can follow A_Str8 sugestion and once in the PC, copy EXIF data from JPG to DNG.
But IMHO the decision of "keeping" is less regretable than "erasing".
