Plaque and dental decay can be a problem for all sorts of animals, not just humans. The last thing you want for your ferrets is toothache from abcesses and the suchlike. Its not much fun for us and we've got cutlery to prepare our food with, your ferret has only got its teeth!
If you solely feed your ferret kibble then they will probably develop a layer of tartar build up over a couple of years or so.

You could try cleaning their teeth yourself with pet toothbrushes or even get your vet to do it but there is another way. Natures way. If you feed your ferret raw whole food such as baby chicks, rabbits, good quality roadkill etc you will find that in no time at all the bones feathers and ligaments being chewed will have cleaned your ferrets teeth back to their natural state. Make this a regular thing rather than a special treat and their teeth will stay in prime condition all the time.