Draw, like everything else in pipe smoking, is highly subjective. If you are sipping your pipe slowly, and not attempting to suck the smoke out of it like a milkshake, then a difference of 1mm probably isn't going to have a significant effect. That said, pick whatever feels good to you. My completely baseless thoughts on this: those who tend to be frequent puffers, or those who take long draws tend to care more than those who sip and/or breath smoke. I can smoke through a 9mm filter and not really notice draw restriction(however I usually don't due to flavor disturbance), mostly because I am not trying to pull air through the draft fast enough for the filter to to be a restriction.. Not to say that frequent puffers are wrong for being what they are. To each his own. :D
I am in slight disagreement with those who say that you can modify your packing or just tamp to tighten up the draw. Yes, this works for the draw, but it can also have a pretty detrimental effect on burn characteristics and moisture accumulation. Some blends taste better with a little moisture, and packing them tighter may result in very poor burn and over-numerous relights which increase heat and contribute to a swamped out heel and shank.