The answer to this is subjective, but the risk you run by not having your appliances sit on tile is that they will be uneven with the rest of the tiled floor. You can use risers to elevate your appliance to the level of the tiles, but risers can shift, and when they do, you will have to move the appliances out of their place to fix the risers. If you are careful when replacing the appliances, you can lay vinyl, stone, ceramic tile, or porcelain tiles underneath with little or no risk to the finished product. That includes keeping the tiles safe when kitchen cabinets, appliances, and even a heavy bathroom vanity cabinet are placed on them.