I have never heard of this for piggies.
I know you can get spot on treatments that you drop on the back of their necks. Is this what the vet meant?
Or an actual injection?
To be honest I probably wouldn't treat regularly for parasites unless there was an actual problem with them.
If your piggies are well and not having a problem, then I wouldn't treat them for something hey don't have.
I guess the exception would be if they regularly come into contact with other piggies that could have parasites (e.g. if you often board other animals which may be infected).
But I am sure someone else who knows more about these things will be along shortly to offer advice.