I am NOT going to claim "guru" status, but here's my thoughts on whether to underscore or not.  I prefer not to, and it's for one main reason, and one minor reason.

Let's say that you use  Home_Address   as your layout element (field) name.  When you prepare any written documentation and decide to use underlining as a means of emphasis, you and anyone else that reads that documentation may or may not see that underscored space:

1)   Home_Address   
2)   Home_Address 
3)   HomeAddress

On #2 above, there is an underscored space, but I don't think it is visible.  (The underscore isn't visible on my preview of this message).  So this can be potentially confusing if you or someone is referring to printed material, and then decides to type  Home Address   (not underscore) in some area of the application to perform an action on that field.

The other "minor" reason...  I'm a decent touch typist, but hitting the hyphen/underscore key without looking is about a 33% possibility.  I consistently hit the "zero", or the "=" keys about as often as I hit the intended hyphen key!