Does the NESC provide guidelines addressing cable sag due to thermal effects?

Yes, in fact the NESC requires the clearances it specifies to be maintained at the “worst case” sag and tension.

Clearances above ground must be maintained at 32°F with ice from the loading zone, 120°F, or greater than 120°F if so designed.

Clearances to buildings, mid-span clearances between power and communication cables, and other clearances in the span also have “worst case” sags specified. For example, some horizontal clearances may specify a temperature of 60°F with a wind pressure of 6 lb/ft².