Peter is right. Not all of the current Nikon Speedlights have an Auto mode. The SB-700 for example doesn't have an Auto mode (other than TTL).
My understanding is, however, that TTL works great with light modifiers though I've never used it myself.
My current flash, an ancient SB-28, does have Auto mode, and it works surprisingly well as a straight flash with the digital Nikons I've used it on. But I stick to manual for shooting through umbrellas and lightboxes. Figuring manual flash exposure is drop dead simple with modern digital cameras. You just take a test shot and check your monitor. No expensive flash meter necessary. Setting the zoom position is pretty much hit and miss. You just have to experiment to find out how to set it for the modifier you want to use. For my purposes I generally have it set to 85mm which is it's telephoto limit. But then I'm shooting through a softbox with a beauty reflector attached which bounces the light to the back of the softbox. I lose a lot of light but the SB-28 is a fairly powerful flash so it's not too big of a problem.