As indicated, you're supposed to provide it if you know it; i.e. if the depositor is across the window you should be able to express an informed opinion. As for the other account holders who might be listed on the same CTR, you can leave it blank if you don't know.
Personally, there is no way I would start accumulating the information at account inception. First, because I don't have to. Second, because it facilitates a review for gender based discrimination; e.g. pay or return decisions on NSF checks, that would otherwise be very difficult.
Hang me if you must, but you're going to need to get your own rope.
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.