to me, my vehicles are all about reliability. i dont use junkyard items like starters, tie rod ends, ect because they already have a portion of their limited life used up. id rather replace those kind of parts with new because then my truck/car is that much more reliable.
i dont even like buying junkyard motors if i can avoid it.
but i would run a junkyard speedo cable. its not a high failure item, meaning chances of finding a good one are high...and chances of it failing after you install it are low. its easy enough to test in the junkyard...just pull the transmission end off and give it a spin..you should be able to see the speedometer twitch.