Phoenix draws most of its water from the Salt, Verde, and Colorado Rivers, supplemented by groundwater, and treats it at municipal plants before distribution. The city publishes an annual Water Quality Report showing test results against EPA limits. In practice, the water consistently meets or beats those legal standards for regulated contaminants. So from a regulatory safety standpoint, it's safe to drink straight from the tap for healthy adults and children.
Safe and ideal aren't the same thing, and the gap is mostly aesthetic. Phoenix water is very hard at 16-20 grains per gallon and runs several hundred ppm in total dissolved solids, which affects taste and leaves scale but isn't a health risk. A lot of residents notice a chlorine taste or smell, since chlorine or chloramine gets added as a required disinfectant. Those qualities lead plenty of households to use a carbon filter or RO system purely to improve flavor, not because the water is unsafe.
A few trace contaminants are worth understanding. Arizona groundwater can carry naturally occurring arsenic, and utilities treat to keep it under the federal limit, though private wells outside city systems aren't regulated and ought to be tested on their own. Disinfection byproducts form when chlorine reacts with organic matter, and nitrate can show up near agricultural areas. All of them get monitored in municipal supplies, but folks with specific health concerns, infants, or compromised immune systems sometimes add filtration as a precaution.
The bigger variable is your own home's plumbing. Federal action levels for lead and copper are measured at the tap, and older homes built before 1986 may have lead solder or fixtures that add metals after the water leaves the treated main. If you live in an older property, running the tap before drinking and testing for lead are reasonable steps. The municipal water arriving at your property line and the water at your specific faucet can differ, because of what sits between them.
For most Phoenix households, the practical answer is that tap water is safe but benefits from point-of-use treatment. A simple carbon filter handles chlorine taste. An RO system addresses TDS, arsenic, and nitrate for drinking and cooking. A softener tackles the hardness that municipal treatment doesn't. If you're on a private well, especially in northern Arizona, regular independent testing is essential, since no utility is monitoring it for you.