I'm pretty sure mine cross, and for that reason I try to keep them rather widely separated. The Rosinas live under the oak tree, and the Sulphureas live in the back under the hemlock, and every time I miss a clump of the locally prevalent non-odorata species near the oak, I find some more seedlings that are dirty purplish pink and short on fragrance.
Things move in anyway, and sometimes they're good - they get moved - and sometimes they're crummy - and they get composted. The only ones that get to stay under the oak, besides Rosina, are tiny little white ones which don't seem to cross with anything that I can determine.

Adam, it all depends how closely related the types of violet are, how close they are to each other, and when they flower.

V. odorata cultivars will cross with each other, though some seasons are more successful than others; the chasmogamous (open, petalled) flowers are not nearly as good at setting seed as the cleistogamous (petall-less) ones later in the year.

V. sororia and many other stemless blue US violets cross much more readily with each other. And they will also cross with odorata on occasion, which is something of a surprise since they are not that closely related.

Hi,

An update...haven't received the viola odorata I ordered for this spring yet, but some of the violas I had in pots last year have seeded into the sitting area around where the pots were. It is quite charming. I do see the colors I had last year, and a few that might be crosses. Very interesting and fun. Looking forward to the new odoratas too.

Adam