I would say toning has a lot to do with how the previous owner stored the coin. A lot of old timer collectors put Silver coin in Dansco Album which contains acid and other chemicals. After that, the toning will show up sooner or later.
I bought 2001-2009 Panda BU coins from an Ebay seller in 2010, who stored all the coins in a Dansco album. I took all of the coins out as soon as I got them (no toning shown at that time), and put them in capsules. now after 4 years, most of them have developed toning at the edge. I also bought the entire set of Kooks from the same seller who sold them in a dansco album too, same thing happened. Although 1 got toning as soon as I got them, so I returned it. And now after 4 years or so, most of kooks also got toning at the edge.
Even though I got excellent deal in buying those, but now all the toning ruined the most of the value out of these coins.
And at the same time, I bought 10 2010 Panda and also some 2010 kooks from individual sellers, who kept the coins in the original Mint capsules. NONE of those got any toning. This tells, it has nothing to do with my storage. It has everything to do with the previous owners putting them in Dansco album that contains Acid and other chemicals