I must have one of the most eclectic model-engineering libraries going… Having already journeyed down the garden this wet morning to find my Zeus book to answer a thread about threads, I have just gone to the workshop again to see what Eclipse had to say on the subject.
This is in a booklet James Neill & Co published undated but some of the drawings, of saw-using chaps in lab-coats, suggest 1950s or early 60s. It was even printed in England!
Naturally it assumes Eclipse blades in Eclipse frames, but the general point is to take up the slack then tighten three full turns only. (I think we were taught that in school Metalwork.)
Relaxing the frame after use though?
The booklet says nothing about that.
It was probably written for trade users who might be expected to wear out a saw frame in as little as ten years (they are not always the tool-caring types we might imagine). However, I think the experience Diogenes gives, and no doubt common to many of us, is due to the fastenings and frame holes wearing rather than the frame stretching over time. I have a saw like yours, and with lots of washers under the wing-nut, but I think because the bore through the handle, and other parts, have worn enough to swing the also-worn pins closer and off-axis.
So I think slackening the blade does not achieve anything useful – as with any tools, even good-quality hacksaw frames and blades only ever tensioned and used correctly, will wear out eventually, but I doubt they stretch.
I'm sure a new hacksaw of good make, with good blades tightened properly, will see us out!