I have never heard anyone say to underexpose chrome by one stop in general use. For some scenes/subjects, yes, that might be appropriate. But if there is a general rule of thumb for exposing chromes other than box speed, it is to underexpose by 1/3 or maybe 1/2 stop. If there are very dark areas, then yes, you will lose more detail in those shadow areas. But the thinking is that highlight detail is better retained, especially on very brightly lit areas of the scene, and it is preferable to slightly underexpose overall than to have blown highlights.
I have a subjective feeling that modern chrome emulsions have more dynamic range than previous emulsions, hence shooting at box speed is "safer". In the past, I would expose Kodachrome II @ 32 rather than 25 (again, dependent on the scene), but K25 was more amenable to shooting at box speed, and similarly K64 can be shot at 64 while KX was better shot at 80. Again, just my experience. And shooting current K64 at 80 doesn't hurt much and yields some increased saturation, if you want it.
As for Elitechrome 100, the last role I shot I rated at 100, and bracketed a few shots to 125. Both worked very well.
In limited experience, I have found that I didn't need to underexpose Velvia; box speed seemed to be fine.