Until Pete Seeger and his book, it was almost always the index. Every "period" account or instruciton book prior to Seegar indicates to use the index.

Many fine players use both, or switch, or use middle for the first string and index for all others. You'll use both if you start incorporating “clucks” as well.

Like you, I started Clawhammer using my index finger. After some time, I switched to my middle finger because I got a clearer, stronger note. Let your ear or your physical comfort guide you. 
 

Some players, like Abigail Washburn, use both index and middle fingers. Middle for the first string and index for the rest of the strings  

David

Thanks y’all! The proof is in the pudding I suppose. I tried some drills and exercises and even though it felt pretty natural to switch to the middle, no matter how I changed the angle of attack so to speak, I just couldn’t get the power and clarity of the index. I’m so new to this, but clawhammer banjo has become like oxygen to me. I need it at all times!

I switch when I injure my index (it happens). Doesn't make a bit of difference to me.

I play the first with my middle finger and the others with my index finger I use my thumb on the inside strings probably more than most, fwiw. This is all personal comfort. Whatever approach gets the results you want is the right one.

I use index, middle and a lot of drop thumb to get the melodies I like to play. Kind of an upside down three finger approach.

Joel makes an astutee observation:  The literature does indicate the use of the index finger, and a lot of the iconic old-timmers used the index as well.

That being said, I've always used the middle finger.  Like David, I get a stronger, clearer note that way.

Additionally, I most easily get the percussive "cluck" when desired using the middle finger hand configuration.

Switching just means you'll be able to use either finger. That can be useful.

FWIW I favour my middle finger for the same reason mentioned above by R.D. and David.

People's hands are as different as their faces. I think it comes down to bone, muscle, and nerve supply as to which finger you end up using. banjered

quote:

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Originally posted by dbrooks

Like you, I started Clawhammer using my index finger. After some time, I switched to my middle finger because I got a clearer, stronger note. Let your ear or your physical comfort guide you. 

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It's funny, I have the precise opposite experience: I play primarily with my index finger because I get a clearer, stronger note. I switch to my middle finger for a more mellow sound. Though from people I talk to, everyone seems to have your experience and I'm the only person I've seen with mine. I wonder what I'm doing differently.