I am importing 350 contacts into Civi. This post says that you should always include an import identifier. I don't have admin access so I cannot create a custom field set. I was going to identify the contacts by tagging them all. Alternatively, Civi includes a contact source field. Could I include a value for contact source in the CSV file? I will also tag the imports.
These are all valid options! I never use a custom field for this; I use tags or the contact source field. The "external identifier" field is often useful for this - I'll tag contacts from a certain location with their ID from that source prefaced by an acronym for that source.
As someone who works daily with data migrations, I rarely need to do any of these things if I'm the one who might have to deal with mistakes. Using advanced logging is good enough; if it's only 350 contacts, it's probably enough to just go by creation date.
That said, I highly recommend that people who don't do this regularly tag their contacts or use the Source field :)