Although Bill Monroe always had a fiddle player, and so did Flatt & Scruggs, I believe it is optional in the history of bluegrass.

The Stanley Brothers, the Country Gentlemen and many other groups stick to a basic four instruments: acoustic bass, rhythm guitar, banjo and mandolin.

There have been some truly great fiddlers in bluegrass such as Kenny Baker, and starting in 1956 the Dobro came into common usage thanks to Buck Graves.