Good morning Jack the answer is there really is no right way. You will find there are two ways normally used and that is as you realize is with thumbs on top or using finger tips.

One thing is using finger tips most often requires the use of some sort of suspended transmitter tray and this for some proves to be cumbersome at times.

In the US the most common method is thumbs. The opposite may be true in some parts of Europe. Also it is some what activitiy motivated. For example trays and fingertips tends to be popular with pattern flyers as well as turbine flyers. The opposite is true of pylon racers as that activity is somewhat of a physical nature and the trays would be just to cumbersome.

To muddy the waters even further there are some that use fingertips but no tray but I would not recommend this to anyone just starting as there is a danger of literally dropping the transmitter.

John