To be honest, it doesn't make much of a difference whether you treat well kept guinea pigs with a high dosed product whenever there is an outbreak or whether you treat with a low level product as a prevention. Many guinea pigs come with mite eggs in their skin; these are not reached by preventative treatment, which only kills the emerging mites.
Mange mites typically hit when a guinea pig has a lowered immune system either from a stress/bereavement or after a major illness. Wehn treated promptly and correctly, they are usually more in the way of a nuisance and not a big issue. I only treat whenever there is an outbreak and have so far always been able to get on top of it quickly.
It is of course a different situation in rescues that take in guinea pigs from a background of neglect. These piggies are usually treated for mites and fungal as a matter of course upon arrival. Many rescues also administer another course just before rehoming to bolster their piggies during the stress of moving into their new homes and lives.
