Re: backing accordion while singing or while the fiddle is playin'
Seconding effectively behind singers and other instruments in a band setting is all a matter of being tasteful, see Jimmy Breaux videos for examples, and an exercise in keeping it simple. Just remember that it's always easier to do too much than not enough and realize when its better to just hang back and let the other musicians do their thing.
I've been in a handful of bands where the accordion player thinks its his duty to play at all times and as a result of them either being deaf or wanting to be the center of attention at all times completely ruin songs by overplaying, playing too loud, and in many cases playing the wrong notes entirely while other instruments are taking their ride.
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