There are no known health reasons to neuter a male guinea pig. My guinea pig has symptoms. Sometimes a simple medical procedure can clear up a problem that would otherwise be fatal. That said, here are some common symptoms with what illnesses they may indicate. As with sneezing, some scratching is completely normal.
Guinea pigs spend most of their time grooming themselves.
However, if the places being scratched are becoming raw or sore, or losing their hair, the scratching is probably excessive. Your guinea pig may have some kind of parasite, such as mites, or fungus, such as ringworm. Bare patches of skin are commonly caused by fungi and can be treated with a suitable anti fungal dip. Many cavy owners use Selsun Gold as the first line of defence. Ask your veterinarian if there is a bad problem.