List Import as Lead Source

Here's how we do it (and i'm a former SFDC marketing employee, so heavy influence from my days there)

Lead source is a one time only field. It's how they originally came into the system. If it's List Purchase, then so be it.
Other examples are Web Referral (for inbound web leads, form fills, etc), Events, Paid Pipeline (for 3rd party form fills, etc), Partner, etc...

Then, we have a Lead Source Detail field that is a free form field to describe more detail about the source (e.g. what list was purchased, what event, what paid pipeline program, etc)

Then, it gets fun.
We have 'Lead Source - Current' field that changes everytime they come back into the system... shows their last activity.
we also have a system in place that tracks what happens when someone goes dark for 30+ days and then comes back. We don't change the Lead Source, but we instead create a new 'Lead Source - A" and then B, etc... for after every 30 day pause. 

But to answer your question, a webinar isn't a source, it's something that a person wants to go to.  That's not the source of your lead (unless it was some 3rd party form fill).  source is a basic, high level field that should be kept as such... then create fields to dig down into the details...

That's an interesting take. I've struggled with webinar because it never quite made sense as a channel since I push people to that with email or web forms, etc. Wouldn't that mean live events and white papers are also NOT Lead Sources and instead are campaigns/programs?

I've usually seen Lead Source used as "Channel Source" - in other words, what channel did they come in from the first time?

-Third Party List
-Events
-Ads
-Social (sometimes broken down by platform)

I keep it as high level as possible. If I want to track specific programs or events, then I tag that person with an SFDC Campaign, which Marketo does automatically, of course. So if I get a registration list from a trade show, I will tag any NEW leads as "Trade Show" and then mark them in the campaign or Program as "Registration List" or "Met at Booth".

Melanie,

Just thinking out loud here, but I was having the same kinds of questions as we stand up or instance:

1- Either lock Lead Source (in Admin panel) or create a Lead Source Original and lock it.  This becomes the how a lead first came to your attention field. Also have a filed that indicates Lead Source Current...as in what have they touched most recently. Now if they are completely new the Lead Status aligns with this list, otherwise the field doesn't change.

2- Create some of Event - Campaign (Program or Content) as a placeholder.

3- Import your list with "Lead Source" named with the Event - Campaign category (not the specific event, just the category)

4- Run a smart campaign-smart list to tag the leads as participating in the Event or Program. Now you know that they have touched this content so you don't send this to them again.

All leads end up being tagged as being involved in that particular content-event.
Unique leads get their Lead Source from this activity.
Existing leads maintain their original Lead Source but get Lead Source Current updated.