As far as I can see:
* Sekihara et al. 2004 calls it optimal
* Fieldtrip just calculates this when the fixedori parameter is True (but they say "compute the leadfield for the optimal dipole orientation" in the comments)
* Nutmeg calculates this orientation by default, so no parameter is used and no comments are made

I'll have a quick look through more papers tomorrow, but "pca" sounds good to me.