from my experience with nukes it doesn't matter a whole lot other than the damage is devastating. here's what i do. i have two distinct lines of research that i pursue, those that allow icbm and those that allow sdi. these two are foremost techs to acquire. once MP is completed somewhere (i never build it), i start building icbm's all over the place and continue until i can no longer stand it. on average i wind up with about 4 per city in my most productive cities. i avoid war at all costs until my sdi is up. i also take pride in using them in anger only once. i've been nuked in 3 other games but most have fizzled out from the sdi. i used to wait until nuked to nuke back. from seat-of-the-pants feelings, suffice it to say that nuking all an opponents large cities makes the takeover of that civ a cakewalk. there is no counterpunch to an invasion and very meager city defense units left. this makes conquest way too easy. i prefer to let them nuke me while my icbms lurking in their bullpens only as a last resort. then i punish the aggressor by humiliating them with land forces alone. I also do my best to estimate where other civs are in their icbm development. i've never been the target of more than 12 a turn, 3 of which actually hit. if they are building them, i invade as soon as possible and build up to a point where i can take most of his major cities in a turn or two. i've been at war with civs that have icbms but they don't seem to use them unless they're starting to lose the war. maybe i'm misinterpreting this but i've gone several turns letting the ai whack at my ground forces with his and this seemed to satisfy his desire enough not to nuke. once i start to roll and take several large cities, it's then that i see the nukes fly. i've seen them target cities with the most units in them on my home land (a city with maybe 5 cruise missiles and a few mech inf). meanwhile, my entire army is on their continent wreaking havoc and not targeted. the land war is a lot smoother knowing those big birds are there for backup if needed.