Metal plates under SPD cleats

How critical is it to use the small metal shim that can go between the cleat and the sole of the shoe? I had a pair of Giro MTB shoes with carbon soles and they seemed to get indented where the cleat was touching. Just retired those shoes and got a pair of Shimano XC-7s. Looks like the sole in that area isn’t raw carbon so it might be ok. I’d rather not use it if possible as I feel like it made the shoe feel loose in the pedal (can’t describe it any different, it just didn’t feel right. Hope I explained this ok…Thoughts?

I might be wrong. But I think the plates you’re describing aren’t meant to go between the cleats and the sole. They’re meant to go inside the shoe, with those nut- type plates, for those shoes that don’t have threaded cleat holes in them.