the tank will get longer while it's off. 2-10mm. I had my tank off a couple years ago while doing a lot of work through the winter, maybe 6 months, drained of gas, and it went back on fine, so they do move back some. I had it off this spring for one day and it lengthened about 4mm. Likely depends on regional fuels. I've used wood blocks as leverage, a big phillips to align the bolt hole, it's a pita but has to be done. The po of my parts bike welded the nut mount back further to the rear. Some use a ratchet strap, which I did last time and it worked ok.
All in all I'd say my tank is not as stretched now as it once was. Weird I know, but I can tell from the distance at the seat body front mount. It used to touch there years ago.