In a manuscript, how do you know when you need to source the brand and location for materials?
It should be the case that material are not dependent on brand and location, but in many cases, especially the biological sciences, they are.  This is wrong and bad, but is also simply the state of the science at present, in areas that have not yet had standards and mechanisms for certifying those standards.  For example, something as apparently "simple" as DNA synthesis has no established quality control standards, so when somebody uses DNA supplied by an industrial supplier, which supplier can really make a difference!  The same is notoriously true for reagents as well---to the point where people will often put down not just the brand but the batch number.

So, how do you know when you need to reference the brand name?  There are two answers to this which are, unfortunately, somewhat contradictory:


Scientifically, you need to put it down whenever the expected variance in materials from supplier to supplier is might make a significant difference in the results that are observed.  If you don't know whether it will make a difference, you need to put it down.  Note that this is also strongly dependent on the size of the effect that you are measuring: if you predict and observe a function with high precision and minimal variance, then the materials are much less likely to matter than if you are observing a 10% variation in a noisy system and had to apply a significance test to determine that the result actually existed.
From a cultural perspective, you need to put the brand name down anywhere that reviewers in your field will expect to see a brand name, whether or not it is appropriate.  You can learn this by looking at papers, but it won't necessarily tell you what the scientific truth is, because there are often unsupported assumptions embedded in this cultural practice.


The conservative approach, of course, is to give brand for the union of both, and that's a reasonable strategy.  As methods progress, however, we should move toward the point where the mechanisms and experimental methods are well enough established that the effects of the incidental experimental circumstances can be minimized...