If I could only insulate in one place it would be where the heated space meets the frost line. I always spec. insulation on the outside of the foundation wall from grade to below the frost line minimum. Where you question points is a tricky but important spot (especially in the frost zone) and normally is address by insulating the edge and cutting at a 45° angle at the top to hide and protect the insulation.

Suggest you research the Barrier at www.underconcreteinsulation.com as this product will do exactly what you are most concerned about.  I have attached a pic for you to view of a project showing how it was applied . 

XPS is the industry standard.

Depending on where you live, you may want to also consider laying XPS horizontally at the footer, 2' wide. This helps move the frost line outward. See http://www.cchrc.org/ Also, there is nothing wrong w/ 2" or more of XPS while you are at it (if it's cold there).

Best insulation system I have used is 2 inch rigid pink Dow board over entire slab. This is real insulation R11, and the pipe then can be stapled to the foam, easy to install, On a 4" slab we would rip 2" X 6" X 8' to run all the way around the perimeter to minimize horizontal heat loss and infill with 2x8 sheets. In colder climes it is effective to also insulate the exterior perimeter foundation prior to back fill. I have seen some run 4 foot out horizontally from the perimeter as well.
Dan