[Tex/LaTex] Oldstyle numerals or lining numerals with units of measurement

Question

What is considered proper typography regarding which set of numerals to use with units of measurement?

A real-life example:

In the experiment, shown schematically in figure 2.2, we illuminate an array of 10 slits, each 50 nm wide, with a laser beam at λ = 830 nm.

Which numerals in the above sentence should be set as oldstyle (text) numerals and which should be lining numerals? As far as I know,

• "figure 2.2" should be oldstyle in any case.
• "10 slits" should also be oldstyle.
• for "50 nm wide", a case could be made for either one. Personally I think oldstyle looks better here.
• for "λ = 830 nm", the answer should be the same as "50 nm wide". But this is even more of a grey area, since strictly speaking the "830 nm" is part of an equation. If it were "f/2 = 50 Hz" then I would definitely want the "2" to be a lining numeral.

Also, is using the siunitx package macros for every instance of a number followed by a unit of measurement the best way to achieve this?

ADDENDUM

Unfortunately using siunitx gives me the dreaded Too many math alphabets error and I'm not wasting any more precious hours of my life fixing that one. Here are the macros that have always worked well enough for me:

\newcommand{\unit}[1]{\mbox{$\;\mathrm{#1}$}}
\newcommand{\micron}{\mbox{$\;\text{\textmu m}$}} % special case of \unit

Answer 1

Personally, I prefer to set any numbers with units as maths, leading to something like:

In the experiment, shown schematically in figure 2.2, we illuminate an array of 10 slits,
each $50~\mathrm{nm}$ wide, with a laser beam at $\lambda = 830~\mathrm{nm}$.

It makes things consistent: you shouldn't use old style numerals in maths and you should include units as much as possible. Something like this looks a bit confusing to me:

The siunitx package just makes it easier to be consistent:

In the experiment, shown schematically in figure 2.2, we illuminate an array of 10 slits,
each \SI{50}{nm} wide, with a laser beam at $\lambda = \SI{830}{nm}$.

Now, this might also look inconsistent - first there is an old style 10, and then there is a lining 50. In these situations I prefer to just write 'ten' or not to use old style numerals at all.