I wouldn't - it's not about the pigs, it's about the people. Guinea pigs like being in a familiar environment, not travelling in small carriers, being in a strange, noisy place with the scent of lots of other guinea pigs (never mind the exposure to parasites and infections). Then sitting in a little pen or on a board and being examined.
People who show restrict the number of times they show any one pig in a year because they acknowledge it's a stressful experience for the cavy. And stress as well as being unpleasant, also affects health and makes animals more prone to illness. So why do it at all? It's not like they gain anything from it.
There are 'pet classes' but most all showing revolves primarily around breeds. And breeding is about producing particular physical characteristics without regard to how those characteristics may affect health, or the fact that we already have so many homeless guinea pigs in the world so don't really need them to churn out more.
Please remember that this forum is not breeding and showing friendly, so while a discussion of it is fine, this isn't the place to champion showing.