I use classicthesis for my thesis, which is quite uncommon for my institution (the LaTeX class provided by the institute has line lengths of 130 chars and very tiny borders on A4 paper).
My supervisor says I can use whatever I want, but I want to explain my choice somewhere in the thesis, especially for the co-readers. I don't want anyone to say "he is just increasing page count with this ridiculous amount of whitespace on each page".
I currently put it into the colophon, before the technical details.
It reads something like this and covers around 80% of one page:
<Why typography is important to me>
<Why I made this choice>
<I hope you (the reader) likes it>
<Technical details (real colophon)>
Is this a reasonable choice?