Wooden chews aren't necessary, and brushing their teeth is something I couldn't even imagine as possible without veterinary tools and anesthesia.
As long as they have unlimited grass hay (and aren't overfed veggies, too many veggies can lead to less hay eating, 1 cup per guinea pig per day of veg is a good guideline) then that is all they need to keep their teeth ground down and healthy in most cases. The silica and coarse fibres in the hay plus the grinding motion of chewing keeps their constantly growing teeth in check.
Most teeth issues in guinea pigs you will see are either caused by a period of not eating enough hay, or by conditions with a possible genetic cause (such as elongated tooth roots).
