Dwarf mondo grass and creeping thyme are the two best choices for planting between pavers. Dwarf mondo grass gives the classic tidy green joint, stays under a few inches, and tolerates shade. Creeping thyme takes sun, handles moderate foot traffic, and releases scent when stepped on. In cool, damp climates, Irish and Scotch moss are the soft, mounding alternative, and blue star creeper covers fast where summers are mild.
True lawn grass is usually the wrong plant for joints. It only works where the gaps are wide and sunny, and it needs edging and trimming forever. Most tight paver joints get too little light and water for turf to look good.
Match the plant to traffic (thyme and dwarf mondo take footsteps, mosses prefer light use) and to sun. To see how a planted-joint patio would look on your own hardscape, OutdoorBrite renders paver and planting combinations from one photo.