Your chickens will graze on the grass. They will make some dust bath craters where ever they choose to. Got mulched flower beds? They have their own ideas about landscaping. They'll fling that mulch across the lawn. They'll dig up your flowers. They will prefer to dig your favorite ones, and preferably the ones you spent the most money on, and most recently planted. In the garden, they'll dig weeds and veggies alike. They will munch your veggies to the ground. They will pick holes in every tomato and every squash. The stuff they don't dig, eat or otherwise destroy, they will poop on. For these reasons, lots of poultry keepers:
Keep the chickens in the run, except for supervised outings.
Fence in the garden or any flower beds that they don't want the chickens to destroy.
Only let the chickens into the garden before planting and after harvest.
I love chickens, and especially love the amendments to the soil, the weed and insect control they provide, the tilling they do, and the entertainment, eggs, and meat they provide.