Well, technically, it’s a choice, but really, do dimple your tie.
In ties, a dimple is an indentation of the tie’s fabric formed directly below the knot. It is a common mark of a well-tied tie. The dimple can both complement a symmetrical knot (e.g. half-Windsor, Windsor) and add balance to an asymmetrical knot (e.g. the four-in-hand).
I dimple all my ties. It’s not particularly hard, especially when I use a thicker tie (e.g. wool, silk knits) or one over 3 inches wide. In the last step of forming the knot before tightening, I pinch the side of the tie with my thumb and middle finger while pressing down on the center with my index finger. The dimple forms, as a result, wherever the index finger presses.
Many well-dressed individuals wear their ties with a dimple, but a dimple is by no means a definitive feature of a well-tied tie. A dimple is nice, but it shouldn’t be forced. On skinny ties, a dimple is difficult or impossible to create simply because there isn’t sufficient fabric in the tie to provide for one. Also, dimples are often hard to maintain on thinner, softer fabrics. If you’re wearing one of those ties, and you cannot form a dimple by pressing on the middle of the tie below the knot, don’t “force” it by over-tightening the knot. The dimple is meant to be a natural occurrence, at best created with a little guidance. If your tie really doesn’t want to dimple, it probably was never meant to do so in the first place.
For the same reason, don’t worry too much about the number of dimples or its (their) location(s). Unless you’re using a half-Windsor or Windsor knot, both of which demand centered, symmetrical dimples, the location of the dimple doesn’t really matter. A dimple of any kind will be sufficient to give “balance” to the ensemble.