theres a huge different in jettision policy of normal day-to-day routine and actual combat.
When you read the pilot accounts from DS F-15 pilots, you learn that they generally jettisoned the external tanks before an engagement, at least the wing tanks first, the centerline depended on how the situation developed. One Flight was engaged by SAM, and they jettisioned the wingtanks, out-maneuvered the SAMs, continued the mission, and then dropped the centerline tanks before engaging Iraqi aircraft.
From reading various books about F-105's in SEA, it was standard procedure to drop the tanks before bombing the target.( edit: apparently not.. see below.. )
For the MiGCAP/Escort F-4 flights, it was the same. The big 600 gal F-4 centerline tank was a ferry tank by design, with speed and G limits for jettision.
It also restricted the use of the 2 forward Sparrows, since they can collide with the tank.
I guess all F-4 pilots just liked a clean plane while fighting MiGs...
In the 1973 YKW, IDF/AF pilots on CAP jettisioned so many fueltanks after GCI vectors which often lead to unsucessful intercepts that they were told to drop them only if they had sight of the enemy.... they run low on extra tanks so fast that many replacements they used were still painted in red primer.. no time for fancy paint.
Soviet and Soviet trained pilots were told by their GCI what to do and when to do it, that included when to drop the tanks...