flip once is the preferred method in every kitchen i have worked in. Flipping steaks early is just like flipping a burger early. It won't cook properly any more on the cooked side when you flip it back. it's been sealed and the heat does not permeate into the meat as it should and would if it had been allowed to cook properly the first time. so what happens is you keep flipping the darn things over and over trying to get it to cook through and eventually dry it out by overcooking it. try it and see, slap a burger or steak down and just let it sear for about a minute on the one side, then flip it, when the other side is done try flipping it back over to finish off the seared side. you'll end up with dry overcooked or will have a raw spot.
don't let people confuse you either with the idea that if they sear a roast and then bake it it cooks properly and doesn't dry out. it does, but that's more about the long cook times that roasts have in an oven and not the fast grilling associated with steaks an burgers.
I have always viewed multi-flippers as inexperienced cooks, they can't relax and let the grill do it's job, patience on the line is hard, especially when everyone from the chef to the busboy is yelling at you for one thing or another.
I have to admit that, not being (even close) to a professional cook, my advice might be so so, but Gunnar, I'm not so sure about the logic (although the process might be right). Searing doesn't affect how much moisture goes through (or out) the meat, the concept of 'sealing' is bogus. You sear a roast simply to add flavour. I would imagine (although I would have to check) that the same goes for the thermal properties, i.e. a cooked surface will let heat through about the same as a raw one.
Having said that, the seared side will cool again, so when reheated it will take some time for the heat to reach the centre again, thus overcooking the outside, but it's more a question of heating/cooling/heating, than the properties of raw vs cooked meat.. Not that I've tested this last argument, but it's what makes sense to me..
The meat will actually tell you when it is time to flip. Presuming you start with a clean, well seasoned grill/grate at a nice high temperature the meat will stick until it is ready to be flipped. Nudging it's side gently to test it's cling will let you know; if it slides, it is ready to be flipped.
Once it is on its second side I find it is best to leave it alone until it passes the finger-thumb test. Touch your pointer finger loosely to the tip of your thumb (on the same hand) and then poke at the fleshy area at the base of your thumb. The denseness, or 'give' of your muscle there is the same as a steak should feel when cooked to rare. Your middle (bird) finger touched to your thumb will give you the texture of a medium rare. Your ring finger is medium, and your pinkie will give you well-done.
If you are really keen on cross-hatch grate marks I suppose you could flip two more times. Just remember that the meat will take heat out of the grate during cooking (as well as warming to a less contrasty temperature) so it'll take a bit more to get the second set of marks. So four flips max, for the pretty. If you flip more than that, that's just silly. Stop that. /img/vbsmilies/smilies/rollsmile.gif
I pretty much do the same. On the grill and don't touch it until about half way, then a 90 degree rotate for grill marks on the presentation side. At the end of the time, I flip and don't touch it again. I think too frequent flipping inhibits that nice char that gives it so much flavor.
Flip once is definitely better. The more you handle a piece of meat the more chance there is to lose flavorful juices. And don't EVER, use a fork to flip meat. Each piercing with a fork results on further losses of the juice.
Once one side is done, flip, when the second side is done, eat it.
The only pierced hole allowed is that of a thin-stemmed digital thermometer.