There is one tiny, possible reason to take note of the location of an enemy nuclear reactor, but it has nothing to do with impacting his ability to research or produce nukes. The AI seems to put a higher priority on defending, and if necessary retaking, provinces they own which have special installations in them like reactors or rocket test sites. So such sites can be viewed as pocket magnets, which is to say you can expect the AI to focus more units defending them and thus they make good targets for you to encircle in order to catch bigger clumps of units to be cut off and destroyed in situ. I am not 100% sure this is true, not being well versed in decyphering lua files, but that seems to be the tendency based on my own anecdotal experiences. But this is a relatively minor point, all things considered, since the AI is already pretty easy to outmaneuver, making such tricks a teensy bit less than necessary.