(Yet another depends. It depends on the type of plant it is ...)
Depending on the type of plant it is, the grass may 'rob' the plant of nutrients and water. But on the other hand, it may not.
I wouldn't use potting soil. If you have, say, a soil that clay or clay-like, you need to 'loosen' it up so water doesn't run away from the plant. Simply bring a good sized sample of the dirt w/you to the Nursery or Home Center and have them look at it to see what you should add to it. Sometimes, all you need is sand or Vermiculite.
Now, about the plant and grass ... Start with cutting back the grass to the same diameter as the plant. From then on out, you need to figure out if your plant is an leaf-line feeder or a stem feeder (when it comes to rain water.) If your plant is the type of plant that 'channels' falling rain water to the OUTSIDE of the leaf-line, then you need to cut the grass just past the outer edge of the leaf-line every late Spring. When it rains, depending on the plant, the plant's leaves may guide the water to the outer edge of the leaf line. This tells you that the root system is a periferal system. If the leaves channel rain water to the stem, then this tells you the root system is a vertical system, and you needn't trim back the grass-line every Spring. See what I mean? This principal should dictate how much of the grass you need to take out.
For example, we have Star Gaze Lillies. The leaves are about 20 degrees from vertical. WHen it rains, the rain water is, literally, collected by the leaves and channeled to the stem where it runs to the base of the plant and into the ground. (This is a stem feeder.) We also have Minerature Red Maple Trees. Their leaf system channels the water to the OUTER leaf line, thus dripping to the OUTER edge of the plant onto the ground. (This is a periferal feeder.)
Another general rule of thumb is as follows: Make a mental picture of how 'wide' your plant will be when it's full grown. (Keep in mind that many plants should be pruned at some point. If they aren't, you'll most likely end up with a wild bush, and NOT a plant!) Anyway, Azellia bushes are usually grown to be about 3-5' in height and 3-5' in width. When you plant this particular bush near the house, or something like it, make sure that when the bush is at your desired size, it's 2-3' AWAY from the house. (The math is as follows: 5' (MAX Width) divided by 2 = 2.5' +3' (from the house) = Plant the bush 5.5' from the house.) By planting in this manner, you allow air to flow around the house, thus, keeping it dry. It helps in avoiding all the problems associated with constant, damp soil like mold, mildew, termites, and so on. PLUS, it leave you enough room to 'maintain' the bush. When planting 2 plants or bushes next to each other, the same rule of 2-3' should be maintained. (Of course, this type of rule doesn't apply to plants like pansies, tulips, and so on.)