There are many customs, lores and matters of principle in grilling, including the sanctity of a burger on the grill. You'll be told by many to only flip it once, halfway through cooking, but it doesn't measure out in taste or consistency.
J. Kenji Lopez-Alt, the Serious Eats blogger who applies a scientific rigour to delicious matters of interest, debunks the "only flip once" idea in a roundup of six food myths he's discovered to be patently false. He tests the theory himself, takes pictures and measures out his results:
Flipping your meat multiple times produces meat that's noticeably more evenly cooked (there's about 40% less overcooked meat in a burger flipped every 15 seconds vs. one flipped once), browns just as well (just don't expect distinct hash marks), and to top it all off, ends up cooking in about 2/3rds of the time. Faster and better? You betcha!
You can look through the results at the full Serious Eats post and relieve your memories of being scorned for your nervous grillside ways. If you've got your own proof of the single-flip system, we'll definitely hear you out in the comments. (Oz ed note: remember, this is a rule for burgers, not steaks!)
The Burger Lab: How Often Should You Flip a Burger? [Serious Eats]