I will start by saying expansion and contraction on the windows installed in a home is less of an issue than windows not properly shimmed to keep the jambs straight. I would make certain the units are properly shimmed per the manufacturer.

Wood does not change length appreciably with moisture content.

It changes width and thickness in a typical piece.

The windows do not need any room top expand in the rough opening.

Installed properly, you should not have any issues.

Proper shims and spray foam are your best tools in this application.

thanks everyone for the replies and assurance i was not missing something. I guess the 1/2" extra dimensions in the rough openning to window size spec is to allow for the rough openning to be 'rough' and therefore not as precisely square as the window, as well as to allow for sealing (with insulation or foam, i'm intending to use mostly foam).

I guess after installing hardwood floors throughout my home over the past year, i've been so focused on the expansion gap necessary at the walls, that not having space for the expansion in the windows seemed strange. However, flooring of course has a lot more wood surface from which to expand.

thanks again.