When installing pads, you will see anti-squeal shims and paste-on compounds. Do you actually need them? Often yes - they target the vibration that causes squeal. Here is what to know about anti-squeal.
What anti-squeal does
Brake squeal is high-frequency vibration between the pad and caliper. Anti-squeal shims (metal/rubber backing plates) and paste compounds damp that vibration so it does not ring out as noise; see brake pad shims and why brakes squeak.
Shims vs paste
Many quality pads come with shims already attached - if so, you may not need paste. If your pads have no shims, a thin layer of anti-squeal paste on the pad backing (never the friction face) helps. Do not double up excessively.
Hardware matters more
Anti-squeal helps, but fresh clips and proper lubrication of the slides and contact points do the heavy lifting for quiet brakes; see quiet noisy brakes and which grease where. Anti-squeal is the finishing touch, not a cure for worn hardware.
The bottom line
Use the shims that come with the pads, add paste if there are none, and always fit fresh hardware and bed the pads in; see breaking in brakes. Start with quality pads and hardware.