Mxsmanic wrote:

Out here in the west, it always seems like I'm bumping up against the
oxygen altitude limits if I'm IFR though...

Why only when you are IFR?

I note that a lot of the areas in the western U.S. have sizable mountains.

You answered your own question; the difference to simplify a bit, is that
the minimal altitude required when flying over mountains is much higher when
flying IFR than what you can do VFR.

If you have a choice between continuing straight on and climbing several
thousand feet to clear some mountains (with the potential need for
supplementary oxygen), and taking a detour to go them without a change in
altitude but at the cost of extra time and distance, which do you usually
prefer or consider more prudent?

Depends on the weather, the aircraft I am flying, whether it is day or
night and my currency (recent flying in the conditions considered and
in turn how confident I feel doing the thing), the duration of the
flight (how tired will I be), etc.

For instance I am fine with flying over mountains, OR at night OR IFR
(in IMC that is); I am getting less confortable with two of these elements
together, and I won't go with all three. Having the choice between single
and (one) multi engine make the choice a wee bit more complicated (well,
actually no, it doesn't; considering the aircraft from which I can choose
it makes more sense economically to pick one of the single engines is
nobody else is willing to share the cost) :-)

A typical flight I have done a couple of times is flying from San Jose
to Las Vegas; I don't mind going over the mountain with a clear weather
and little wind early in the morning; but I'll make the detour southward
when flying back in the evening/night.

May be am I just a wuss,

--Sylvain