My personal experience with tipi-sag has been that it's minimal, only slightly bothersome, and easy to remedy. Almost any woven fabric will have some stretch and movement in the weave, and will relax after being pitched a day or two. Wind, rain and temp fluctuation may make it more noticeable, but never severe for me. My center pole rests on a flat stone. If I notice enough sag to warrant an adjustment, I simply raise and lengthen the center pole via the push-button adjuster, which tightens the pitch and removes any sag. I usually do this only once on a 12 day Alaska hunt with temps from 10F to 70F and everything from bright sun to cold rain and snow. I have never had my tipi sag noticeably at all when correctly pitched. It goes without saying that a bigger tipi will be more likely to experience some stretch after pitch.
I personally prefer fabrics which have some stretch and give in their weave. This reduces stress on seams and pressure points (cones, stake loops, etc) adding to the overall life of a tipi. I've seen the videos where correctly-pitched tipis withstood near-hurricane winds, while floored tents were damaged by wind getting under the tent and lifting it. Tents and tipis need to flex and move some in response to wind gusts, or else damage will occur to fabric or structure. This has always been a basic tenet of mountaineering tents exposed to huge and nonstop winds.
I might be interested in a non-stretch (and ultralight) fabric option, but not until I had seen proof of it's long-term durability in all weather and wind. A doubling in price would need to be accompanied by major benefits immediately, plus over the longer term. I'm not a strong doubter...I'm just one of those guys who let others assume the risk while I watch for feedback.