Re-tuning the probe (correcting both tuning and matching) is essential, but does not necessarily restore the rf field to the amplitude obtained with a non-conductive sample. In my experience, conductivity affects the 1H channel most significantly, and the effect on 13C, 2H, and 15N is negligible. The effect scales with frequency, so I would expect changes also for 19F, and perhaps 31P to a lesser extent.
The relative loss of rf amplitude depends on details of the probe. Probes which apply a relatively high rf electric field to the sample (this depends on the coil design as well as the circuit design) will have higher sample-dependent losses. Probes with very high Q (e.g., cryo-probes) will probably be more sensitive to the electrical properties of the sample.
Here is a specific example: A cryoprobe with which I am familiar has a 8 microsecond 1H 90 degree pulse with non-conductive samples (e.g., chloroform in acetone-d6 standard sample). With salty protein samples, the 1H matching needs a large adjustment, and the 1H tuning just a little. The probe tuning and matching controls have sufficient range to fully correct the tuning (so the reflected power is negligible). Following that, the 1H 90 degree pulse is in the range of 10 to 13 microseconds, depending on the sample composition. 90 degree pulses for the other nuclei appear to be unaffected by the sample.
