Not so long ago iPhone development was quite simple, only a few OS versions and even less devices.
Now however, there are 2 major OS versions and 5 different devices to
I'd suggest a couple things:
you probably will need a range of devices for your own internal testing. As has already been pointed out, there are various differences between each generation of iPod Touch/iPhone
you may want to look into using crowd-sourcing to supplement your internal testing. This potentially allows you to have a bigger audience to test your apps and any cost of using crowd-sourcing is potentially offset by the fact that you can spend less on devices and potentially less on internal testing staff and at the same time, potentially producing a better product. One crowd-sourced test group not mentioned above is uTest.com