The higher you go, the more noise you will get in your threshold, and a noisy threshold will mean missing those faint signals or minor hiccups. They will be masked by noise, and differentiating between the two if you can even hear it would be near impossible.
It has and will always be a rule the cleaner you keep that threshold, the more likely you are to hear abnormalities in it.
If the ground permits, as Heatho said above, then by all means run it as high as possible. But it most cases, 9 out of 10, the threshold will be eratic, and that is a no no.
Good luck