Personally I worry more about overtightening/stripping plugs like that than undertightening. (Race cars can vibrate plug loose, hence torque wrenches or wiring are necessary.) I make sure to use small hand box-wrenches for jobs like this, rather than automatically pulling out my 1/2" drive ratchet set. Mush less likelihood of overtightening that way. I'd simply use the proper (maybe 14 mm?) box wrench and tighten it reasonably hard, but not "bust a gut" hard.