I think you could draw a distinction between *necessary* and a best management practice. Necessary for low pressure? No. Good idea? Probably. For example, in many areas, you can't use clear primer because inspectors want to be sure that you used primer (purple or blue). This is more of a necessity in high pressure scenarios, but it does soften up the PVC and make a better/stronger bond between pieces. Throw my 2 cents into the mix.