I always change them all no matter what (if its a broken string, or a dirty one). The high E is always gona give you the most problems, so thats usually the one your going to need to change the most. The low strings almost never break since they are way more durable. So if you start taking singe strings out of packs, your going to have a bunch of left over larger gauge strings. Might as well throw them all on at once and have a brand new set.

If your in the middle of gig and a string breaks however, changing them all isn't practical, so you want to just change the broken one and be done with it asap.