The important thing is that the pressure is on the sides of the shaft. The CCs are side by side in your penis, you should be trying to deform them with your grip but still allow blood past your grip as you progress up the shaft. People do use top to bottom pressure and get results, most notably with a power jelq style device but the top of the penis is more delicate and top to bottom pressure works the CS harder, so even this type of device might be better used vertically.
If the soft area between thumb and forefinger is located to the top of the shaft and you exert the pressure on the sides, that should be fine.