Solid Hardwood, Engineered and Laminate Flooring - Fasteners and Adhesive for 4"+ x 3/4" hardwood plank?

I'm looking at installing 3/4" solid hardwood plank and think I have narrowed my selection down to 2 manufacturers. The two I'm down to are Somerset and Columbia, both in Hickory in the 3 1/4" to 5" wide range. While looking at Somerset's install recommendations they advise using both adhesive and fasteners for 4"+ plank and Columbia doesn't seem to have any reference to using both. Is this just an extra preventive measure or am I inviting trouble if I use fasteners only or would that be determined by my subfloor? My subfloor is 1/2" ply over diagonal 1/6s on 16" OC joists.

Also, how well do the sound abating underlayments decrease noise to the floor below when used with 3/4" solids?

The wider the plank, the more cupping can be seen. Gluing the edges, has been known to resist slight cupping.