You can only use the lowest common temp. rating, including terminations, per 110-14(C).
For instance, feeders would include OCPD temp rating at main switchboard/panel, conductor temp. rating, and temp. rating at lugs at subpanel. SWBD and panels are typically 75°C per the listing info on the panel covers, and THWN is 75°, so 75° can be used.
For branch circuits, the OCPD ratings are 75° or 60°, per what's printed on the device, the conductor rating (usually 75° for THWN), and the utilization device (usually 60°); so you usually have to use 60° for branch circuits.
I rarely ever see a combination of conductors and terminations that allow the use of 90°.

[This message has been edited by energy7 (edited 11-30-2004).]