I think it is important to learn to use rhythm expressively without losing control of it. Teaching specific details, like agogic accents, tenuto, tempo variation are important. The beat has to be constant. Rubato in the usual sense is tempo rubato, slowing or speeding. A little goes a long way, and harp rhythm is heard differently from piano, it seems. We need to have much clearer control of the beat and any modification of tempo. The momentum is important, if you slow down too much over one expressive figure, you lose the momentum toward the next phrase. I think a student should only learn what is written by the composer in expression until it is memorized and well learned, and then modifications can be explored. It will help to listen to other music of the period and genre, particularly other Russian composers. There is no strict answer to this unless the Russian tradition has carried one down through time. In the period in which it was written the left hand would keep fairly strict time and the right hand might vary from that. Slight ritardandi at ends of phrases or sections may be appropriate.