I think the simple answer is when whatever alternative techniques you use do not yield sharp results. It is worthwhile to take a comparison shot with all parts of the tripod locked down, mirror up (if your camera supports it), no hands release (if you can), and with the tripod out of the wind or other sources of vibration. You appear to have done that.

I personally find I can shoot at 400mm with reasonable shutter speeds (1000th) on a monopod, easily on a tripod, without noticeable shake. With a TC at 800mm, I find I can rarely get a shot without some blur on the monopod even at much higher shutter speeds, and even the tripod takes some significant degree of care (I do this a lot shooting baseball from the center field fence). This will vary by your own steadiness, sensor density, wind, tripod stability, shutter slap, etc.

I guess I am not too surprised at 500mm that just ordinary care yields sharp results; you are probably just steady. Try using a 2x TC on it, and you may find your limits somewhere up there. If you are just looking to experiment, just try dropping the shutter speed down to 100th or so, and see if you can see the difference when using different techniques. Also, try putting the lens hood on and going out with a good stiff and variable crosswind some day; I find shooting wind surfers to be particularly trying in that regard, those big lens hoods act like a sail themselves.

But... the short answer is, it only matters when it matters. I know that is not terribly helpful, but the reality is that all the various components (including you) can make a huge difference in when "good long lens technique" really starts to show (or lack thereof).