Do I need to rebuild indexes when upgrading a DB from 2008 to 2012?
I found interesting this link: http://thomaslarock.com/2011/12/7-mistakes-you-cant-afford-to-make-when-upgrading-to-sql-2012/

In my case I upgraded from sql 2000 to 2014 (with a 2008 bridge) detaching and moving files, but considerations are still good. My db whas about 1.3 terabytes.

I run

DBCC CHECKDB WITH DATA_PURITY;


But this is critical just becaming from 2000 or 2005, not your case.
Anyway: if you are moving datafile, you must check db... It is an online operation.
Also:

DBCC UPDATEUSAGE(db_name);


You can skip this.

Reindexes? I didn't find any documents about it. In my case it was even not acceptable. To much time and resources.

I updated statistics. I think this is important when you move a database. This can take long time, yes, but database is available to users. It is mandatory? No, but it makes me feel safe...