Hi @Dave0600 - Great question! A new tent fly should be good to go without adding any additional waterproofing.
That being said, an extra layer of protection can feel reassuring if there are rough weather forecasts ahead. If that's the case, this spray-on treatment is a great option. There are even some comments about using it on tents in the reviews!
Hope this helps!
Let me also suggest that you do not need to waterproof the fly prior to use. A good tent should be waterproofed and have its seams taped. If you are still uncertain, set the tent up in the backyard and turn the hose on! I often try out new equipment under controlled conditions to see how it performs.