Commonly used? I don't think I've ever once heard the idiom without "down," and it sounds plain wrong to me without it.
Eh, it's a big country and everybody else talks funny at times. You require it, I don't. We could both easily find a dozen people to agree with us that the other is wrong, just by going out to buy some milk.
One thing you may have noticed or surely will notice soon from time on this forum is that there are frequently disagreements about the acceptability of a word or phrase between even native speaker members of the same country.
Isoglosses and dialect boundaries change sometimes with as little as a 15 mile geographical difference. Sometimes the size of the country attributes to the linguistic dispersion, but not always.
For instance, look at the UK versus the US. The UK is about 1/40th the size of the US geographically, and still has something like 56 regional dialects, while the US for all its greater size, only has roughly 24 or so. The exact numbers vary depending on what you consider distinct (versus sub-dialect) and whose research you cite, but I think it's rather fascinating that a country forty times smaller has over twice as many dialects of the same basic language.
With that in mind, I'm rarely surprised to hear another American disagree with me about what's acceptable. It's only a surprise if/when somebody who has a similar background and upbringing as myself disagrees with me.
Here are a couple of sites showing a few of these regional AmE differences. I've reviewed them probably a hundred times and still find them fascinating.
(with additional AmE dialectical information elsewhere on the site)
Harvard dialect survey