Answer to your question yes it will fire ItemCheckedIn event, if inside Versioning settings link->The Require Check Out radio is selected.
Some more information:
1. once we add document, automatic checking will happen, which will fire ItemCheckedIn.
2. Next when we explicitly call ItemCheckedIn.
following sequence you may notice in event execution.
1.User clicks on the Add a Document link
2.SharePoint displays the Upload a Document dialog
3.User uploads the Document to SharePoint
4.SharePoint fires the ItemAdding Event
5.SharePoint adds the document to the Library
6.SharePoint fires the ItemAdded event
7.SharePoint displays the property editing screen to the user
8.User edits any properties they wish to change and clicks the Save button
9.SharePoint fires the ItemUpdating event (first time it is fired)
10.SharePoint sets any document property field entered in the property editor dialog
11.SharePoint fires the ItemUpdated event (first time it is fired)
12.SharePoint now begins the process of automatically checking in the document
13.SharePoint fires the ItemUpdating event (second time it is fired)
14.SharePoint fires the ItemCheckingIn event
15.SharePoint fires the ItemUpdated event (second time it is fired)
16.SharePoint fires the ItemCheckedIn event
Hope this will help.