First, you never know what your needs will be. This team looked set at LB just a couple years ago, for example. Needs are constantly in flux.
Second, if a QB (or some other position) is BPA and you really don’t think you need that position, you absolutely look to trade down. That’s pure BPA strategy.
Third, I can’t think of a position that might be 1st-round BPA this year and not be a position of need for the Cowboys. An RB isn’t going to be BPA. Neither is a TE. Neither is a QB, really, except that they always get overdrafted. Is there another position where a top rookie couldn’t get tons of snaps on this team?
Finally, in later rounds, the differences between players are so small that it’s a non-issue. You might have 5 guys who are all “BPA” at any pick, so take whoever you feel like. That may be a decision between positions, or between ceiling vs. floor, or something else. On the other hand, if someone has fallen and is clearly the best guy out there, grab him or prepare to regret it.
The whole “well acshully you shouldn’t do pure BPA” thing is incredibly overblown. The Cowboys seem to put together a very strong draft board. They should stick to it and pick the BPA or try to trade down, period. (Or trade up to get the last guy in a tier on their board).