Yes, 10% or less will generally keep out out of trouble. With the 1K/100K combination, the source impedance is 1% of the input impedance, which is well under 10% and isn't going to present much in the way of issues.
In fact if you have the headphone jack of a phone connected to the input of a Crack, the ratio may be more like 10R/100K.
You can get into problems when you have a source component with something like a 12AX7, even when wired as a cathode follower. In that situation you can end up with a source impedance from the 12AX7 that looks great at about 700 ohms, but questionable operating current to drive a 7K load, and things get even more dicey when you add interconnect capacitance and input capacitance of whatever it might be trying to drive!
As far as audible issues go, you can end up with treble roll-off in the scenario above. If you took something like our Eros or Reduction phono preamps and fed them into a preamp with a 10K input impedance, you'd get a drop in gain, an increase in distortion, and some low frequency roll-off issues.
