Guinea pigs of both sexes can be neutered, and in some cases
     should be.  There is some evidence that neutering a female can
     reduce incidence of uterine cancer.  Also, if you have an older
     female that may not have had a litter yet, she should definitely
     be spayed for her safety.  There are no known health reasons to
     neuter a male guinea pig, although I hear that it can reduce
     their sex drive and cause them to stop mounting female guinea
     pigs, if that is an issue.

     The risk can be very small if you find a good vet to perform the
     surgery.  You can begin by looking for "exotics" (read: not just
     cats and dogs) vets in the phone book.  Or call ordinary vets and
     ask who they refer their serious guinea pig cases to.  Call
     around, ask for recommendations, and don't be afraid to drive a
     long distance -- it's only once, and it could save your fuzzy's
     life to be at an experienced vet's.  Ask any prospective vet how
     many guinea pig spays/neuters they have done in the past year,
     and what their success rate is.  For a good vet, it should be
     well above 90%.