Tracy at Rabbit and Guinea Pig Welfare in Rugby has all the piggies (boars and sows) neutered before rehoming. The vet she uses is Simon Maddock and he is so experienced that a guinea pig spay is no more dangerous than a castrate op. However, very few, if any other vets are as experienced as Simon and the operation can be very risky.
I've got a spayed sow who lives with Archie, one of the dental piggies and she looks after him so well. Archie has so many health issues that I would never have wanted to put him through a castrate op, so getting him a spayed sow has been ideal.