You should stick Arial at the bottom if your first-choice font is something similar in x-height and design sensibility, because it will make your site stay more consistent in design. If your first-choice font is a completely unrelated sans-serif, then there's no reason to put Arial at the end.

Thus, you’ll notice that in this particular article, Arial is at the end of the “Helvetica” stack, but not at the end of the “Gill Sans” stack.

Also, DejaVu Sans Mono is never going to keep design continuity with the site’s intended sans-serif font, because it’s monospace.