I don't like knots for terminations when a splice is possible. But sometimes you have to do it. I agree with TheTreeSpyder that strength is not a worry. Mike Maas brings up a failure mode that I wasn't aware of.
But there is another failure mode that is maybe less obvious. I learned of this on a rock climber newsgroup. Someone decided to test some of the knots that climbers frequently use, especially the bends they use to connect long pieces of rope. They will actually hang from a rope that has a knot in the middle, which we don't do. The result was surprising. One or more of the commonly used bends would imperceptibly creep every time it was loaded with 150 lbs or so. The creep was about .001 inch per cycle. Even after 10 or 20 cycles, you would be hard pressed to see any change. But the experimenter kept at it, and after 1000 cycles, the knot had unrolled a whole inch with no change in shape or apparent security.
One of the most highly regarded bends used in the climbing world is the figure eight follow through, considered to be absolutely bullet proof. You could use this as a termination knot, but I don't know if it is any less prone to creep than a double or triple fishermans. Bottom line: if you use permanently installed knots, as on your lanyard prusik cord, inspect regularly! For a one-time use, as on your climbing line, just tie it well and forget about it.