Importing contacts into Civicrm
These are all valid options!  I never use a custom field for this; I use tags or the contact source field.  The &quot;external identifier&quot; 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 :)