The closest I get to skin stretching is turning my head to the side; rolling my lip over; ballooning my cheeks, etc. Never found the need for more.
What's your practice?

Ditto.

I use my off-hand to stretch my skin continually throughout the shave when using either a shavette or DE. It makes a difference for me in terms of closeness and comfort.

Yes. A flat, unmoving surface yields a more efficient shave. Seems a no brainier to me.

The older I get, the stretchier I get.

But to answer the question; yes, I do some skin stretching with my free hand. Usually more on finishing passes.

My beard tends to lay pretty flat and my face is getting flabbier so it helps to create a proper surface.

Same, unless shaving with a straight, in which case I rub my free hand with an alum block and stretch all the skin except for the upper and lower lip.

All the time. I use alum only finger tips to get a good grip. It made a big difference in my shaves.

Mostly I tip my head back, move my jaw, or make shaving faces.

I get by fine just using the contortions for my face. But my neck needs all the help I can give it.

I was blessed with tight skin, even at 54 yrs old.

That’s what Mickey Rourke says

Yeah, but I look better than Mickey.

Same here, but i only have one hand that works. While I've learned how to move my head into different positions to gently stretch or flatten patches of skin to make them flatten out. I know hold my breath or puffing up my cheeks works, but rarely use it.
On a side note, if an area is problematic. As in the spot under my jawline on my right side. I'll close my eyes and concentrate on the feel of the blade. With a straight I'll touch down with a knuckle, then bring the blade into contact with skin. It's actually a bit harder with a de. I'll usually keep eyes open until blade is on skin.
But stretching can be very useful to make a flat surface to shave.
I think sixgun said that if your searching for the bbs, stretching may be counter initiative. By stretching, you actually change the angle of the follicle. When you release, the follicle pokes back out. So you can stretch, but the best you'll get is an ok result.

Show off...

That's not too difficult.

Interesting. If I were to stretch the skin on my neck, I would never get it shaved smoothly, because the darn hairs grow almost parallel to my skin!

Mickey looked better than Mickey.
Tempus Fugit. Morietur puer, bona relinquere cadaver.

Skin stretching is definitely recommended in this case.

Skin still tight at age 69. I never needed any "adjustments" on my face.
As for my neck, all I do is tip my head back like I'm a Pez dispenser.

I don't buy "my skin is tight" as a reason not to use two hands and stretch. My skin is tight. But unless you shave with two hands your tight skin still isn't held as firmly in place as it is when stretched and the shave therefore not as efficient. Whether you "need" that additional efficiency is your own call.