Ditto to C.F.'s remarks.
Unless it's an obvious signal i.e. a penny or fuzzy foil signal, I think you'll find that most beach hunter's dig almost all of the targets anyway. I'm not sure how many rusty nails are on the beach's where you are but if there are only a few of them, I wouldn't be too overly concerned with digging a few of them. I always detect with no discrimination and as mush sensitivity as possible on the beach. I ignore all the penny signals (around 35) and unless it's a 38-39, I'll move on. Even then, aluminum cans can give you a silver (39) signal but it's easy to swing a sand scoop and check the target to see if it's a silver ring or a piece of silver jewelry, medallion etc;.
Obviously, when you go too close to fire-pits or spots where people randomly light fires in the sand, you run the risk of detecting masses of melted and buried aluminum beer and soda cans and junk but also nails and metal staples from burnt wooden pallets. If I want to detect around those areas, I might consider lowering sens; and detecting on the surface. I usually don't go near the fire rings though because it burns up valuable time that I could be covering the rest of the beach.
All that said, with deeper targets, the beach can give you some weird signals especially in wet or 'black' sand but as a rule of thumb, dig all unless your sure it's a target like a penny or those fuzzy aluminum and foil signals which are plentiful. Sorry to ramble on! lol.