I try to run through mental blocks - sometimes running is more mental than physical. If i'm just having a rough run - not painful, just having a hard time getting into it, i try to run through it. Changing pace (slowing down/speeding up, intervals) can sometimes be helpful.
If I'm having physical pain, I try to evaluate what is going on (is it something that will go away if I stretch vs. an actual injury). I've had DH pick me up from a run because I was in so much pain. Pushing through soreness is one thing, but I try to be smart about pushing through actual pain.