Whatever is comfortable, but being able to feel what the kite is doing is important. Try gripping any way you like. depends on your hands, the strength of the wind, what the straps are made out of, etc, etc.
I use wrist straps probably 90% of the time these days but very rarely put them around my wrists. I like to hold them in my fingers, pinching the straps with thumb and forfinger for control and feel. The material the straps are made of is most important. I have several pairs of these cheap straps which came with cheap line on cheap kites. They are a very soft, thin material. As you can see, they are getting pretty frayed, but are comfortable. Other "better" straps are often thicker or harder and not as comfortbale for the long haul. I used to use finger straps held the same way, but almost always the seams and such end up rubbing on my first finger. The JOE finger straps shown are probably the best Ive used. The stitching isnt too abrasive. For years, my favorite straps for low wind flying were made from loops of a very soft rope I found at the supermarket.
I've found that if the wind is too strong to hold th straps in my hands rather than around my wrists, I should probably stop flying, or just take frequent breaks. These days the straps only go around my wrists when I'm taking pictures or otherwise need the use of my hands while the kite is in the air.