I have a very similar situation with my chickens. I actually was ignoring the sneezing because everything else with my chickens was so normal, but because of a totally different situation with 2 new chickens that I had in quarantine, I had the Maryland State Vet come out to my farm to do testing. He went ahead and swabbed all of my chickens and my chickens that I had since they were very young all together tested positive for MS or Microplasmic synervy (sp?). They were the ones with the sneezing.
I was told that MS is so common and most backyard flocks actually have it but because our chickens live such stress free lives they seem to do well with it. It does lower their immune system and lowers production, but most backyard flocks would never really notice the loss in production.
Basically I was told to keep my chickens living stress free and make sure everything was always clean, especially the roosts, not to change up their life much and they should live normal lives. If they started showing more symptoms (like a severe cold) I could dose with tetracycline to reduce the symptoms but they would never be cured.
I was told not to be crazy with bio-security when I left my farm because really everyone else probably had it (except for true commercial chickens).
Funny thing-- my flock recently kicked out a rooster-they were mercilessly picking on him - and his sneezing became awful. I pulled him from the group and he lives separately with another rooster and his sneezing has really stopped since he is now used to not living in the flock.
Hope this helps.