Changes to Orchestra tuning.

Total speculation but I would expect to use a frequency that is as much in the roundhouse of as many instruments as possible.  2 hz is not a big change though I would expect if so it was determined to be a better fit.  You can look up music frequency charts online that will show you the harmonic frequency ranges of common instruments.  I have one on my wall.  It comes in very handy to help me understand what I am hearing when listening to the ol hifi better. 

I have on my wall too :).  Bought from the Vancouver Symphony, long ago  :)

Frequency ranges of individual instruments have little to do with the choosing of one tuning reference over another. As has been pointed out 2 hz is a very small change that in no way taxes the capabilities of any orchestral instrument. However, a 2 hz change can subtly alter the overall sound of an orchestra. A 2 hz downward change will cause, particularly strings, to sound slightly warmer. Conversely, a 2 hz upward change will cause the sound of an entire string section to sound slightly more brilliant since the strings have to be made tauter on the instrument. More tension on the strings will cause the instrument to sound slightly more brilliant even when playing the same pitch. Subtle, but not insignificant. Also, keep in mind that it is common, almost inevitable, that no matter what the reference tuning pitch that the oboe gives, as the orchestra plays the “communal” reference pitch will rise. As wind instruments play and warm up the pitch rises. Professional players make an effort to stay at the reference tuning pitch throughout a performance, but it is unusual for the pitch to not rise slightly.  Sometimes more than slightly.

I've never heard an Oboe give the reference. Always the 1st Violin. Am I missing something?

Yes.  

Don’t know what orchestra concerts you have attended, but the oboe always gives the tuning reference in an orchestra.  The oboe sometimes gives two tuning “A440-445”s (or whatever).  One for the winds and then one for the strings.  When it is a piano concerto the tuning reference “A” is taken from the piano on stage by the oboe and then as above.  Only when there is no oboe, in a chamber work for instance, does the first violin give it.