It's not so much whether or not someone else thinks it's ok or not, what you decide is what's important. Any underplanting, even if it's in the form of lawn grasses, will be in direct competition with your tree for resources (water and nutrients). From the tree's perspective, it would be much better if you were to forgo the underplantings (including lawn grasses) and instead used several inches of arborist's wood chips. This eliminates the competition of under-plantings and ensures the tree gets the lion's share of nutrients and water as the mulch breaks down, with the remainder of the available nutrients contributing to increased populations of soil biota and a healthier soil.
So, now you see both sides of the coin and have information enough to make a decision that best meshes with your wants and wishes. Choices are good.
Al
