I don't often handle the uploads of ebooks to distributors personally, but I believe most if not all of the larger distributors (B&N, Sony, Kobo, Apple, Amazon... ) utilize a means of supplying metadata separate from the epub itself. It can be via a web-based interface, or via a custom spreadsheeet, or an ONIX file, depending on the distributor.
This "external" metadata typically includes more extensive data than is included within the epub. E.g. fields like BISAC codes (primary and optional secondaries; both codes and human-readable descriptions), author bio, long and short descriptions, pricing, geographical distribution, and so on. Some required fields and some optional, but everything in a format specified by the distributor.
I'm not aware of any distributor that makes use of the internal metadata in the content.opf of the epub itself -- at least not to any great extent.
And for that matter, reader software seldom makes use of it either (Calibre can read and display many common elements, however.)
So while it wouldn't hurt to put in as many <dc:subject> tags as your heart desires, putting BISAC codes in the epub is a waste of time.