if i go back and update a rate on an IR will NS recalculate average costs and costs on IFs that item might have been fulfilled on?

In our experience, yes. As long as the IFs are in an open period and are dated the same or after the IR date. Other inventory reducing transactions may also be affected such as negative Inventory Adjustment, Inventory Transfer, etc.

okay awesome. i though i remembered somebody telling me it would but i couldn't find it in writing anywhere. it's a daily recalc, right?

I think the timing is driven by a preference. Setup > Accounting > Inventory Preferences > Schedule Inventory Costing

Warning, it can take a while