As mentioned a couple posts above, collateral shouldn't be a compulsory thing one does every time. I only do it if I actually need to. If I can make a decision on disposition/treatment/whatever without it, then what's the point? There should be something specific you're needing from the collateral that would aid in diagnosing, treating, developing a safety plan, etc.
I'd get collateral if the actual patient was too impaired to provide any meaningful history, the patient isn't providing a useful history, the history being given is suspect for various reasons, or the patient is a minor. Since I'm CAPS, I get collateral all the time but it is always done with a purpose.
I see many of the therapists in my clinic obtain collateral out of habit more than anything, which ends up just wasting their time listening to useless information with zero clinical value. I'm not going to call a kid's teacher just to get their side of the story if I think I've been able to get enough from the patient and parents. I will speak with a child's teacher if nothing the patient and parents are reporting makes any damn sense.
It's a bit amusing to me the number of clinicians who do things without really asking themselves why they're doing it and what value it actually has. This way of thinking is how we end up forcing patients at every single psych or therapy appointment to fill out of a bunch of meaningless symptom screening checklists that are really only for the primary care environment and which have absolutely no place in a specialty or subspecialty clinic.