One reason I encourage use of shoulder rest, or something to allow holding fiddle on shoulder firmly by chin, so you could support it solely there (removing left hand all together, carefully at first), is to free up the left hand for things like (potential) shifting to different positions and vibrato(which is easier with a straight wrist, though admittedly not impossible the other way). It also seems to me that fingering can be done faster with a straight wrist, and all the strings can be reached more easily, as well as 4th finger ease of use. I do realize there are fiddlers out there very happily going along with wrists flat against the neck, and instrument held loosely against their torsos, but that seems awkward to me. I do let the instrument slump on occasion, if I'm just doing backing stuff, but when I'm going full out, I need to have a good grip with my chin (though don't give yourself a neck ache or nerve twitch by clamping too hard). More later
Pioden