I never took mine for checkups either, as I unfortunately felt more qualified than the vet (before I found a wonderful exotics vet, but you couldn't do checkups there as it was by referral only).
To counter
@Hannah_xx's perfectly valid point, I've previously found that when I went to a vet concerned about my guinea pig's health, they initially couldn't find anything wrong, and I had to prompt them to investigate further!
If it gives you peace of mind, then by all means send them for a yearly checkup, but definitely use a vet experienced in guinea pig health.
P.S. Apparently yearly health checks are a thing for humans too in America?!