Set to_csv(index=False) by default
By default, the optional "index" parameter of the "to_csv" method is set to True. However, I would argue this violates the principal of least surprise. That is to say, when calling this method I, as a user, would expect that my data frame is converted into a CSV format as is, with no data added or removed that I did not explicitly add or remove myself.

Adding an index column without the user specification is kind of weird, unexpected behavior. Not unlike adding a column for type of reference hashing, or dropping a random column from the data frame without the user specifying this in advance. Thus, I would argue that the "index" parameter should be set to False by default.