One of the first things is to look at which cards can hurt you enough that they are worth countering. For example is you use all 2/2 sites or use 15 locations, then you probably don't have to worry about Revolution. If you don't use Obi or Luke then Courage of a Skywalker and Run Luke Run! probably isn't worth it. However, if you know of something that will destroy your deck than you may want to protect against it.
The second consideration is based on your experience will you see the cards that can destroy your deck. This goes back to "meta", where it depends on what is big where you play. If you haven't seen a Rebel Barrier since Dagobah then you probably don't need a Blast Door Controls. If however everyone and their brother uses Bacta Tank and Savrip then Shut Him Up OR Shut Him Down and Masterful Move will probably come in handy.
So, you put these two considerations together to determine whether or not a "Magic Bullet" is worth it. If you're playing a LS MnT deck and have lost to HDADTJ a lot then I suggest you start using anti-dueling cards and cards like Transmission Terminated. Very often I have won games and known that 2 copies of one card would've meant I would lose. Magic Bullets were designed to punish people for using the popular strategies, use them! Cheers, Jon Schultz