Q:

What is a suitable map projection for the entire state of Idaho?

I am working with some sample GIS data for Idaho. The data is in geographic coordinates but I want it in projected coordinates.  
I am unsure how to address the issue of projection as Idaho crosses UTM zones 11 and 12.  
Should I:

apply UTM 11 to all?
apply UTM 12 to all?
apply another system completely to all?
filter the data into those that fall into 11 and 12 respectively and work with them separately?  

A:

I would advise against using coordinates outside UTM boundaries as this can cause unforeseen problems further down the line when others use the data and may be unaware of this. I would consider it bad practice unless no other alternatives exist. 
This has been recognized as a problem in Idaho and a solution exists so why not use it? The Idaho Transverse Mercator (IDTM) statewide protection was built for this purpose. It is SR-ORG:6677.
Your data is unprojected and you need to select a projected system. Why select one that does not cover the state when a perfectly good one exists and was designed just for this purpose?

