So I thought I would follow up on my own post with what happened on arrival at Las Vegas yesterday evening. I had spoken with CBP prior to departure from IAH (Houston) and the officer I spoke with said that for most airports the form was no longer required and everything was processed electronically.
However, for Las Vegas that is a port of entry where the declaration is still required to be completed (either electronically via GE or APC or via the paper form). Insufficient forms were loaded on the plane and passengers were informed that if they needed one then they should fill it in when they got to the immigration hall.
I had completed and printed the electronic PDF copy ahead of time and only needed to complete the value field and sign the form. There was absolutely no issue at all with the PDF copy. The CBP officer didn't question it and I was processed and on my way in moments. Equally the second officer in the baggage reclaim hall didn't query it either.
So, based on a sample size of one, it wasn't a problem. Hope that helps someone else with a similar question.