I use a three part system in the valleys prior to putting down shingles.
First, 3ft of ice and water shield.
Second, 15# fiberglass felt weaved in the valleys.
Third, 20in 26 gauge galvanized rolled valley flashing.
Why? Because call backs get expensive and in Minnesota we get some really nasty weather. Is it overkill? Yup.
Ice and Water shield may not ever be code in your area. If you want it go for it but you may not need it. It’s intended to be used when ice dams may occure at the eaves. It’s also intended to be used in valleys were a lot water will flow and snow and ice will sit for long periods.
The idea of ice and water and then rolled valley flashing is if you cut too deep with a closed valley method you won’t cut beyond the valley flashing into the ice and water shield.
I can honestly say that with the three part system have not had one call back in several years and hundreds of roofs.