Units for storage of internal time zone offsets

We have "always" stored our time zone offsets in a compressed
divide-by-ten form to allow storage in a single byte. But there are a
few time zones ending at a 45 minute boundary, which "minutes divided by
10" can not represent.

But "minutes divided by 15" can represent this and afaik all other time
zones in existance. I have patches which will do this (with changes done
by a perl script so the math must be right ;). Can anyone think of a
counter-example time zone which does not fall on a 15 minute boundary?
Or any other reason to not move to 15 minute quantization for these
values?

btw, the "45 minute cases" are Nepal and the Chatham Islands, and the
"30 minute cases" are New Foundland, Canada and India, a bunch in
Australia, and a few others.

- Thomas