It does not matter where you put it, unless you also have another .entry-header selector somewhere else that also sets text-align: something - that may override it.
I have a system I use for CSS in sites I develop -- I sort it as follows
- Reset Styles (if I use them, I am currently off using them)
- layout styles
- typography (which includes lists and so on)
- utility styles (here I stick stuff wordpress needs (align-left/right/center and friends) as well as any utility styles (.clear) and so on that I use
That way I can quickly find where to look in my sheet, you of course may find something else works best for you...