Hi everyone, great discussion. I couldn't help through in my 2-cents-worth. I live in a sub-tropical part of the East Coast of Australia. I've just completely replaced an old Hoop Pine (araucaria) floor in a 100-year old "Queenslander" house with T&G Spotted Gum boards. We used a PorterCable pneumatic cleater because we didn't think that staples would actually be able to penetrate the super-hard Australian hardwood flooring joists to which the floor was nailed because they would just bend like the little wires that they are. Conversely, the cleats went in nice and deep, and their barbs hold them in pretty well. Now, that's not to say that there was not a LOT of creaking for the first couple of weeks after install, but we had a run of very high temperature and high humidity days, and the "creaking" seemed to be independent of cleat location - so it was rather an artefact of the tongues and grooves - and only present on extremely humid and hot days. It is completely absent any other time.
Having said that, I've not run into a tounge-in-groove floor nailed to joists (young or old) that does not creak. It's not the same as nailing to subloors and in our climate, where we have lots of timber houses on posts high off the ground to let the breeze circulate under the floor, it's unlikely that we will ever have a creak-free floor. Different mindset - we actually think it adds character!
Cheers,