Updating Modified/updated Date Column?

If your tables contain created and modified/updated dates what is the best practice for these?

1. Should you use UTC dates?
2. Do you use a default for the creation date (I assume yes)?
3. Should you create a trigger to handle the last update date? Or do you update the column directly in your stored procedures that modify data?

Also, as an aside if you store the user who created/updated the record do you store a foreign key reference to the user table or do you store the username as a varchar? Of course I know you'd normally store the fk, but I wasn't sure if the "logging" nature of the column suggests storing a string value.