Head bolts installation question

Hello. I have a doubt.. im going to install the new head gasket after a top end job (lapping and other basic stuff) i wonder if i have to buy new head bolts or not. The manual states a bit lower torque if already used ... What your advices and experience about that?
And, the machine shop in order to clean the head surface told me they grind something like 6/10 of a millimeter. Will this affect the compression ratio?
Thanks

Your head bolts should be fine to re-use. Yes, your compression will be a bit higher. Nothing to worry about.