Yeah I use the blocks on both sides. I know it seems crazy that just a little pressure will keep it from slipping but it's not like we're gluing 45 degree angles here. I bet that technique might almost work with that, since the pressure wants to evenly hold both sides of the body or wing to the neck.
Also another trick is don't really crank down any of the clamps until the glue actually starts to tack up a bit. With Titebond it's about 5-10 minutes, then clamp the tinfoil clamps tight and then the body clamps. If I recall the backside both foil things were running horizontally. Of course wipe it off and after let's say 15-30 minutes or so, you can pretty much unclamp and wipe under the foil since it will still be wet, the release foil doesn't stick at all, nor does it let the glue do anything.
Usually at that point you really don't need the clamps anymore, of course humidity and temperature will affect this time line.
For the body glue ups I'm not a fan of long clamp times, now neck lams and fingerboard glue ups are a different story.
Also let me say that a lot depends on the joint between either the tops or wings and body, I use a joiner and that is about as nice as I can get it, I don't think I'd joint them by hand.
I hope this helps.
Dirk