Yes sometimes Flatt & Scruggs played straight time. A good example is Homestead On the Farm cut with Mother Maybelle Carter on autoharp in 1961. That is pretty straight. The first Carter Family cut of the song was in 1929 for Victor and it was also pretty straight with an autoharp break. The second Carter Family recording was in 1935 for Vocalion and it has no autoharp break but it is bouncy.
Flatt & Scruggs and Mother Maybelle followed the straight time of the 1929 recording with autoharp even though it was Sarah who played autoharp on the 1929 recording. Now later on Lester Flatt and the band recorded the song with Mac Wiseman in 1971 and again with Bill Monroe in 1974, both versions were very bouncy.
So the question do you play Homestead On the Farm with autoharp straight time or the Bill Monroe/Mac Wiseman bounce?
For reference I include both Carter Family versions, not bouncy and bouncy.