account_print_invoice.sxw is just a way to edit report.. actual file that does composing the invoice iz account_print_invoice.rml...

After pushing sxw you may experience new layout visible only on newly confirmed invoices (SO's, PO's...)
This is because it is (mostly) not legal to change invoices once printed. So the pdf file is actualy generated only on first Print.
after that the pdf is fetched from ir.attachment ( so you will not see any changes in design until you print out a completly new document )

To override  this default behaviour go to Settings ->Technical -> Actions -> Reports,
Find the report you want to modify ( for example: Invoices) edit : Untick field Reload from attachment, and save.

Now, every time you press Print you will generate new pdf file, and any design changes (pushed as sxw file or done in rml file)
will be visible on next print of document.. 

Do not forget to change it back once you are done with editing design