From a chemical point of view toning will depend on how the coins were stored and the crystalline structure of the coined metal. Generally we are conditioned to think that toning does start from the outside and this is due to the method in which the vast majority of coins are stored:
1. In albums
2. In rolls
3. In 2x2 holders (maybe less likely, I am not sure 2x2's have been around)
In each of these cases the interface between the coin and the chemically reactive holder is around the edges and thus toning is more than likely to start around the edges. Hence the term 'album toning' or 'roll toning'.
However, from a mettalurgical point of view toning can happen in areas of highest stress on the coined metal. In these areas the crystalline structure is finer and with more crystal/crystal interfaces it can result in areas of the coin thatc will tone more than others. Take some images we've seen on here of bag toned morgans, in this case the toning is across the surface the coin and doesn't seem to have started at the edges. This is because the surface of the coins was in contact with a chemically reactive coin bag.
The other factor to take note of here is that Australian pre-decimal proofs were often struck with business strike dies that were hand polished. In the case of the 1955/56 penny proofs this can lead to strange toning and luster breaks due to the die polishing marks in the fields.
To be perfectly honest, with coins as rare as these (and pre-decimal proofs this early are extremely rare) I wouldn't be bothered too much even if they were AT. It will be a long long long time before coins like them come back onto the market.