Based on my personal experience, it appears to me that how coins are stored makes a difference on whether clad coins will tone, haze, or discolor. As I mentioned earlier, clad coins in my collection that I bought straight from the US Mint have not discolored.
I dont' know how dealers store their proof and uncirculated coins, but many which I purchased from individual collectors and some small coin dealers have shown the greatest amount of toning and discoloration with time.
Also, one post recommends that I dip coins to remove the discoloration. I thought that coin collectors, in general, recommend that one should not clean coins. Thoughts?