In salted road country I typically do pads and rotors together, but not always. If there is any gouging, not uniform wearing, significant rust I just swap the rotors too. Otherwise you are going to mess up your new pads. As others mentioned, now a days, its often just as cost effective or cheaper to get new rotors vs getting them machined/turned. If you are taking the vehicle in somewhere, the labor shouldn’t be much more to do the rotors to if getting new ones.

Judging by the pads and rotors (and how they come apart) and how the caliper looks when you remove it and push the pistons in, us when typically the decision on new calipers come in.

So pads and rotors are normally able to be determined by a component person before the job is done, but often times calipers are not until you are there and even then, sometimes it’s a guess until you test drive it. I’m just a home mechanic with a nice garage set up and older vehicles so I tend to do brakes more often than the average joe, especially with friends vehicles, definitely not a professional though.

Trusted independent shops should be your go to especially for brakes if not doing yourself.