There are pros and cons. Pros to grinding are it can help with gulping, bad teeth and old dogs. I don't have a grinder but we do smash some big bones up.
Cons are that whenever you pre-grind, pre-cut/chop/blend you introduce oxygen, leading to oxygenation and long term storage compounds the resultant nutrient loss. Flash freezing helps stop the loss but how many of us can do that? And, if you are going to pre-grind, chop whatever the slower the better. When I looked into the juicer my Vet strongly hinted would be best I found a slow masticating juicier (for the fruit and veg, not the bones but the principle remains) would be best, and best would be to feed the stuff right away, no storing in fridge. The juicer was a nuisance to assemble, slow to use, a nuisance to disassemble and a pain in the neck to clean. I bought a high end blender instead.
However lots of people grind and their dogs are not dying from the reduction in nutrient value. Lots of us blend instead of juice and we are thriving and so are our dogs. I haven't seen numbers on nutrient values of ground meat as opposed to whole (I did find on the juicer vs. blender vs food processor) so I think it's entirely what works best for you.