I'm so used to filtering out questionable characters, but is there any point if I just storing this in a file? I'm not doing anything silly like using the input to name the file, so filtering combinations such as "../" doesn't appear necessary.

This all depends on what you are doing. If this is an interactive script then by all means verify input and request fixes where needed. If not interactive and this data in the CSV is going to be used by another program that does not expect tainted data by all means verify the data and reject tainted entries.

-Waswas