First strike promotions are more effective for siege than for normal units. The model for combat is the units take rounds going pewpewpew at each other. First strikes give you extra pewpewpew turns with no return fire; city raider or combat promotions make each pewpewpew stronger. Siege combats are naturally reduced in rounds (in BtS) because the siege unit can't get a kill - a weakened opponent artificially truncates the combat for siege ... the average number of times the unit's strength is used must be likewise reduced. Similarly, the weighted increase in turns that first strike gives you must be higher because the average number of exchanges has decreased for siege.
In other words, first strike provably gives more bang for the buck to siege than first strike gives to other units. If you like it for other units, you might try it with siege.
On a practical level, I usually mix city raider and first strike (with no collateral or just plain combat) on the others.... if I am leading in tech, more first strike promotions - otherwise, a little more on the city raider. Trebuchets are good candidates because their innate +100% vs. cities further reduces the weighted benefit of city raider.
I have rarely regretted a hybrid first strike promotion set on any good offensive unit. It keeps them healthy longer, it negates others' first strikes, sometimes it will even pull a horse unit (with no defensive bonus) out of the stack.
Do first strikes do collateral?