Re: backing accordion while singing or while the fiddle is playin'
Whatever you do please do not play the melody quietly along with the fiddle or steel. It is easy to overpower other instruments and plus nothing makes the hair stand up on your neck than when the accordion comes back in on time after the other rides. I agree with john and nedro a little in the right place goes a long way.
Re: backing accordion while singing or while the fiddle is playin'
The only time you're allowed to play the melody along with the fiddle/steel is when you've been playing with the fiddle/steel player for more than 20 years