My original bike came with a 9mm stem. Reach way too long. I changed it for a 5mm and rode that for months. That helped the reach, but my steering was noticably wobbly, especially when climbing hills. I was correcting my front wheel like all the time, which was extremely annoying and somewhat unsafe when climbing narrow hills in traffic.
So I changed again for a 7mm, and that seemed to be a good compromise.
Lots of people will say a 7 is too short. It 'is' too short, but it's a good place between too-long and too-short for me. The steering is no longer wobbly, and the reach is better than the old 9mm.