Depending on the look you want, but most of the time yes you do want to use gradients. If you want it to look cartoony then don use any, but if you want it to look realistic you will be using gradients everywhere. As for polyfills you should use them to cover big gaps but most the time a 2-3 triangles or other nodes will fill a gap without as many nodes being used

That gradient is 😬

I know it’s hard to get them in order