Just a month or so ago.
I had the same questions and with the help of this forum took the plunge and went with 3 lens. The stock 18-55, a telephoto 55 to 250 IS and a Fine Forty (40mm) fixed lens.
On the trip I used all three lens but left the fine forty on most of the time for quick shooting. Here's a shot with the 40mm and then a video I used the 250mm but really wish I had brought a tripod.
40mm shot at https://dl.dropboxusercontent.com/u/29864315/Fountain%20Paint%20Pot%20Nature%20Trail.JPG
55-250mm video at https://dl.dropboxusercontent.com/u/29864315/Yellowstone%20Bison%20crossing%20river%202013.MOV
I think the stock 18-55m is nice for most work.