Should thin spaces be used between numerals and units

As I understand the official SI manual (which siunitix is supposed to follow), there should be a space, not a thin space, between the number and the unit:

The numerical value always precedes the unit, and a space is always used to separate the unit from the number. (section 5.3.3)

I also find that this seems to be the more common procedure in typed texts (but that might again be because most people don't know abut the thin space).

Just to be clear, the SI manual is aware of the thin space, cf. the following section 5.3.4:

Following the 9th CGPM (1948, Resolution 7) and the 22nd CGPM (2003, Resolution 10), for numbers with many digits the digits may be divided into groups of three by a thin space, in order to facilitate reading.

And precisely for this reason, the siunitix package has since this question was asked changed its setting from using a thin space to using a normal space, see this update.

You can use either of U+202F NARROW NO-BREAK SPACE or just plain U+00A0 NO-BREAK SPACE, but you certainly should not let it risk being line-broken. I’m not so sure that the thinness matters half so much as the no-break property. You do not want to let the figures get orphaned without their units.