I rather tailor my shipping discount to fit my customer needs.... than using a set fee for additional items.
I would not know what to set that fee to... 25 cents? 35 cents? 50 cents?...
It's much easier to figure it out at the end of the auctions with all the items ready to ship.
For instance, I was able to fit 30 books in a flat rate priority box for $11.00. But if I had sold only 20 books... the flat rate shipping would be the same price...
Again, I rather customize the shipping fee than having a set fee per additional items.