Toejam,
I think the advice you are getting is excellent, however, my experience was a little different.
I did not shadow DOs, I worked for three of them (in the same practice). A completely different DO from another city wrote my letter of evaluation.
One of the DOs I worked for graduated from my first choice school (the one I am going to) and he was willing to write the evaluation, however, an opportunity presented rather quickly to open a practice in his hometown and I did not want to burden him further. I didn't even bother the other two DOs (three grumpy perfectionists... we did not get along all that well :wink: ).
The DO that ended up writing my letter had graduated from a school in another state (which I liked as much as my "local" school).
My point is that I don't think shadowing is all that necessary (at least in our cases), it is good to get a letter or letters from DOs you have worked with, However, it may not derail everything if the letter comes from yet another "outside" DO (or at least in my case it didn't seem to matter).
This is not advice, just giving you a glimpse at my experience. I think the advice others are giving is right on the money.
-JmE-