I can not confirm based on any official documentation or anything, but I can tell you that I too have only ever seen those numbers increase and don't reset until a reboot. It would be nice if Microsoft had some actual documentation on netstat, but alas.
And for what it's worth, on my server, which is very busy, I saw my "Active opens" value at well over 5 million just before I last rebooted it. It seems unlikely (ie. impossible) that there are actually 5 million ports open so therefore it must be a cumulative value.