What;s the differance?

I keep reading people saying to do the upstrums with the flesh of your index finger but I am in the habit of using my thumbnail. Is that "wrong" or does the maxim to do what feels right apply here? Am I missing out on a nice sound?

You got it....do what feels right to you. There are no hard and fast rules.

I used... both ways !
Depends of the song or the general feeling of the moment

Aito

I'm a thumb only player actually, and while I know it's not the "correct" method, it works quite well for me.

When I try and play with my index finger, i sound like I have never played the uke in my life.... oh well, someday I'll learn that.
For now, I stick with using my thumb only.

Whatever works best for you is the CORRECT way to play! Enjoy!
Cheers,
Skottoman

If you really want to twist your head over strums try reading a description of how to do Claw-hammer style and then .....try to do it......you know that thing where you turn your hands upside down, interlock the fingers, then twist your clasped hands up towards you and then choose a finger and try to move and you always move the wrong one? Like that?