DaveJoran, if you installed the same valve stems that your link shows, you don't use extensions. They come with stabilizers for the long stems that fit in the wheel, not the cover and do not need tying. As for the wheel covers; if they fit properly they should not need tying either, but nothing wrong with a little insurance.
My wheel covers fit snugly and are not easy to remove.
The stems are rigid and I did not need too modify my covers in any way.
I did bend the stems slightly to allow my TPMS sensors to clear the hole in the cover.
2014 Solera 24R