The wonderful thing about Marti's templates is that the extra 'corners' have been removed. You know what happens when you sew patchwork; you often have little 'ears' of fabric that need to be trimmed off. These are already removed with the templates. And some shapes are very difficult to piece since you may not be sure where to position the two fabric pieces.
I used two other tools along with the templates when demonstrating the monthly blocks to my students.
Quilter's Slip-n-Grips are sandpaper 'dots' with adhesive backing. I put a few of these on the wrong side of each template to prevent slipping as I cut. They are effective and inexpensive insurance.
The other product I used when cutting around the templates is Olfa's 12-inch square Rotating Cutting Mat. It is easy to hold the template in place with one hand, cut one side and then turn the mat with my other hand to cut around the rest of the template accurately.
Look closely at the photographs of the isosceles triangle and side triangles. You will see that the corners are precision designed so the cut edges match perfectly. You just need to match and sew. Since the corners are trimmed away there is less bulk in your seams and with sharp angles you are eliminating the pointed end of a triangle that is most likely to get knotted up in the throat plate of your sewing machine.