A dimple is certainly not required when wearing a tie, but I think it usually looks better with than without. Generally speaking, the higher quality the tie, the better the dimple. That’s because the materials used, both for the outer shell and inside lining, will affect how well the tie will curve.
I’ve also found that many times, though not always, a better dimple can be produced using the double four-in-hand knot, like you see here on our friend Mistah Wong. Whether you can use this knot depends on how high your trousers are and how long your tie hangs. You always want both ends of the tie to be no further than three inches or so apart from each other, and for the front blade to end at about the middle of your belt. Obviously, these things don’t have to be exact, but those are the general guidelines, and you may not want to have the tie end at your sternum just for the sake of having a nicer dimple. Unless you’re Bryan Ferry, anyway.
(Photo above by Most Exerent)
I like a 4-in-hand too, but for tall guys this is impractical. It makes the tie way too short. Better tie = Cleaner dimple.