BC, Function: abbreviation, 1 before Christ —often printed in small capitals and often punctuated (Merriam Webster Dictionary)
Searching on Google Books I verified, or, at least, I perceived the idea, that the cases in which "BC" came before the date are rather rare respect to the cases where it came after, especially observing reliable sources. Instead results show that "AD" position is more variable.
Obviousily I cannot show precise data because it is not simple to operate sistematic researches with "BC" and "AD". They are little words, abbreviations, rather common in books and, sometimes, with other meanings.
An example, among others, of "BC" placed before date: "Egypt: From the Earliest Times to B. C. 300".
Question is: Is it legal placing "BC" before the date? Or, is it a matter of style and of editorial choices?