I have never heard of putting clear nail polish on the dogs' nails to make them stronger. I don't think it would really do a lot - their nails are pretty strong, for the most part. Especially if they're fed a good diet.
I do nails (with the dremel) at least every two weeks. I can still hear clicking on the floor (we have laminate) but not as much once I trim them. I don't like the nails to get too long, as it can damage the dog's foot.