I use both HD or regular foil, depending on what i think I need. A couple things I do to help prevent sag: I make sure to use a big enough piece of foil so that I can wrap it around the bowl stem so it gets a good grip. I also use a tip of Kalutika's, smear some tobacco juice around the rim. This helps seal the bowl and kinda glues the foil to the edge. Also, make sure whatever you use to poke the holes is sharp so that it doesn't pull on the foil and finally, re-tighten the foil after poking the holes.
One last thing, check out this video if you haven't already seen it. It shows Eric, the guy that invented the bowl, packing Tangier's. The thing I noticed is he pokes holes right in the center. (Fast forward to about 4:30) Can't suck down that way!