Well, I think I know what you're asking.
In Cajun music, the fiddle usually plays the lower octave behind the accordion lead. This is what we refer to as, "bassing", behind the accordion.
I've got to ask: when you state that you are starting a waltz in "E" on a "C" accordion, are you saying that the tune is played in the key of "E" on a "C" accordion, or, are you saying that the first note of the song is "E" ?
Jude

Jude,
On " La Valse de Quatre-Vingt-Dix-Neuf Ans" The first note is a doubled E. The fiddle could bass octave the E as well, or it could play the low C in the same chord. I'm just trying to imagine what is the best sound. Thanks.
JB

On a C accordion, pulling #4 & #5 would give you an F my friend !

I do apologize my friend, #'s 1, 4, 7, 10 on the push on a C accordion is an E and you are correct, the kick off does start with the E note.

Yep, when you pull on 4 you get F, but pushing gives an E. I didn't clarify which direction I was going with the bellows, sorry. We'll just squeak around 'till we get it right.
JB

hi! i think that if the fiddle is tuned in key of c it will match the c button accordian. good luck and never stop playin! phil

Ok, so after all that , we're playing the "99 Year Waltz" on a "C" accordion in the key of "C".
Irregardless of what the first note of the tune is ( BTW, I play the "C" just ahead of the "E" ), the fiddle will start off "bassing" a "C" behind the accordion .
Jude

jude is correct. Normally for most Cajun Dancehall style music, Fiddle starts bassing on the low octave or just playing key parts of the melody on the low octave till it comes time to take a fiddle ride, (usually but not always after the first verse) At that point, fiddle player does whatever the heck pleases him. Usually playing the melody on the upper octave.

Thanks to both Jude & Layne. That was what I needed to know. See the above site for real good tuning and key signature finger positions for a violin/fiddle. Can really only tune a violin correctly one way. Tuning down to F will work, but you really don't get the best hum out of the strings. Got to convince this kid, well, he's 20, but that's a kid to me, that wasting 10 years of violin lessons and a high-dollar Antonio Strand violin just because it ain't as macho as working on a car engine, is losing out on what could be a potential for a lifetime of fun. Like I told his Dad, the girls always take the fiddle player home.

JB