My B17 develops a squeak about once a year. I basically do what Ottawa_Adam said. I apply a drop of light machine oil to all the contact points of the tension adjusting screw. Then I rotate the screw back and forth very slightly (maybe 20 degrees of rotation)to work the oil into the contact points. That usually fixes it.

I have also had the leather squeak at the rivets. (Warning, the following is conroversial.) I brushed on some neatsfoot oil around the rivets and let it soak into the leather. That fixed the problem. I have had to do this only twice in 7 years. Some will tell you that neatsfoot oil will damage your leather, but that has not been my experience. I do use pure neatsfoot oil, not the compound that contains silicone.

My B17 has about 10,000 miles and the leather is still taut and quite hard. I have not tensioned it ever.