For me, "to and fro" usually connotes physical activity whereas "back and forth" is more likely to refer to mental/intellectual activity.

E.g.,
"I ran to and fro, from one house to the other."

"We debated the question back and forth for hours that night."

To some extent the two phrases are interchangeable, but I think I probably use them metaphorically when applied in a sense other than as stated above.