Hi,
One of France's most popular CBA players, Aimable, often used his forefinger only for repeat notes on the same button. However, he had a fairly unique staccato style of playing that would be regarded as very old fashioned these days. The modern set are all thumbs and legato stuff with the occasional button trill (with different fingers), as if to pay lip service to the traditional musette style.
No doubt other older players did the same, but in the absence of video proof I would have to say that Aimable was the only player I've actually seen using that single finger method.
So, if you want to play like Aimable (if you've ever heard of him) then use a single finger, but it seems that the overwhelming majority of both PA and CBA players have been schooled to use alternating fingers. As far as I can remember from the various French and Italian CBA method books I've looked at, where the subject of repeat notes on the same button is covered at all, then alternate fingering is advocated.
If you want to get really fast then try using just your fore and middle fingers. The use of the ring finger in addition to the 1st an 2nd fingers can often slow things down a bit, particularly with the smaller French buttons, if your accordion has those. I've watched some PA players trilling with all 5 right hand fingers, and it is very effective. However, if you try that with the little French CBA buttons you're almost certain to miss the button with one finger and/or struggle to find the correct button for the next note. If you have large hands there is also the risk of hitting two buttons at once.
French CBA with its smaller buttons has its advantages and disadvantages, although I appreciate that most forum users who play CBA have accordions with the larger button size and spacing.
