Not true in the slightest. Most schools weight the most recent performance more heavily, as it is indicative of who you are now versus who you were four years ago. An upward trend looks great, a downward trend looks... Really bad.
This is true with Canadian schools if you had a 100% course load over 4 years. Some will look at your best 3 years or your best 2 years. American schools don't do this.
I have not heard of any dental schools. Though a decent number of PhD and Master's programs do, as they are really only interested in how well you did in your upper div sciences.