Depends on how cold-natured you are...
65-75 degrees, you'll usually see suggestions for a 5mm. It's always easier to flush your wetsuit, if you're too warm, than to warm up, if you're too cold. If you do multiple dives/day or over a few days, you'll chill, and need the extra couple of mm.
A good compromise might be a 5mm Henderson Hyperstretch. The Hyperstretch suits are slightly less thermally insulating than a standard wetsuit of the same thickness, which would be fine for that range of temperature, and they're the most comfortable suits I've ever put on... Very easy to doff and don, and very forgiving if you don't typically fit "off the rack."
The only down side I've seen is that they tend to "pull" a little easier when they come in contact with velcro.