grass roots will compete with your tree roots in a water and nutrient limited situation. But in a well watered pot, well fertilized it is not a big deal, as long as the grass is shorter than the lowest branch of your tree. Grass will shade surface of the soil, keeping a pot cooler in sun. If watering gets missed grass will dry the pot out faster than neighbors with no grass. Generally I'd say no grass except in a kusamono.
I keep grass in with my pots of native Cypripedium, and Habenaria orchids, they encourage the same mycorrhizae that feed the orchids.
With large, omono sized trees, grass indeed has been used by some instead of, or with moss and maybe small ferns as underplantings, But the tree really has to be big for grass to be in proportion.