Which profile are you using? What ever it is, stay away from country manor shake. If you like the cedar shake style, go with rustic shingle, otherwise, like i said - oxford shingle is a great profile.
Classic uses shark-skin, and while it is a good underlayment (much easier to work with it then with tri-flex30) both of them, and you can add titanium and other synthetics, will trap moisture, which gives you a very good chance of growing mold in the attic, even if well ventilated. That is why we use deck armor.
I remember threads here where some roofers said they won’t let their guys work with it “since it is slippery” and some other arguments. The choice is yours, but it works great for us. Their second version of it, they made much thicker, so it could have been the first, thinner type that guys did not like. For us it is not slippery, as my guys walked on it 20 minutes after a light rain, on a 8:12 pitch roof (of course wearing a harness)
Any way… I&W should definitely be on valleys and drip-edge (eaves), also end-wall (roof to wall). We do not use it around chimneys. We do however runt the underlayment 6 inches up the chimney, and make it water-tight even if it rains overnight. Than we use aluminum (or steel) flashing and counter-flashing. It never leaks
When you have a choice, always go with aluminum. It does not rust, so if roof is installed properly, it will last a lifetime. With steel you always have a chance of rust (not to say that it will, but it
Strength of metal panel is not important here as it is not structural. Longevity is a factor. Classic roof costs 3-4 times the asphalt (it is not a v5 crimp panel that any joe-shmo can buy at a local lumber yard), and for the money you should get a life-time roof. That is why we always go with aluminum if possible.