I know the difference between straight time and swing time, although I’ve never dissected it intellectually to the degree that some other posters here have done. I play banjo in a mostly traditional bluegrass band, and pedal steel guitar in a country band that also plays Western Swing. If you are referring to traditional bluegrass banjo playing, I have a different take on your question than that of my esteemed colleagues Ira and Jody-with all due respect to them. A lot of time you want to go with them and other times you might want to take them on a ride into your rhythm. I think you should listen to the rhythm section and see what they are doing. Some great musicians can even switch up feels in the middle of a song. Usually when it gets past a certain tempo that feel goes away. If you listen at Allen Shelton play "Shucking the Corn" on his albums, he is playing VERY swingy at a fast tempo. However, if it's Gold Rush done at the right tempo (I don't like it fast), then it's going to be swung.
Big Mon? It's going to be pretty straight if you play it really fast. Sometimes tempo dictates if you can swing it. I like playing those straight especially if there is a good bass player and we are locked into one another. If it's Nine Pound Hammer or In the Gravel Yard then I try to keep it straight. If it's Little Georgia Rose I might play it swung or straight, it depends on the jam and what the bass/mandolin/guitar are doing.