How far back should one go in citing classic works?
The ultimate question is what would the average well-informed bachelor's or master's degree-holder in your field be expected to know.

Something that comes up in an undergraduate textbook, or is a commonly known and easily demonstrated fact (for example, the definition of the error function), probably doesn't need to be cited, because everyone in your field would be expected to know it. On the other hand, if you're publishing in a "general interest" journal, then you might want to assume a lower "base" of knowledge, and cite a commonly available source.

However, if it's something that's a very specific tool or fact (or a derivation, etc.), then it probably merits a citation in any case.