If you are supporting the weight of the TX with your hands instead of the neck strap... you'll end up using the thimb on top of the stick almost every time.
Glad you said "almost every time".
I've been "pinching" since I started (I hold the transmitter with my palms and three fingers). To me it feels more precise.
Keep in mind though; there are a number of world class RC pilots that fly "thumbs", so if you're more comfortable that way, I wouldn't worry about it.
Dennis-