Spill protection is a safety requirement in many states. Be familiar with the laws in the states you drive in. You should look at your cargo covers in the mirrors from time to time while driving. A flapping cover can tear loose, uncovering the cargo, and possibly block your view or someone else's. You cannot inspect sealed loads, but you should check that you don't exceed gross weight and axle weight limits.
Some containers have their own tiedown devices or locks that attach directly to a special frame.
Others have to be loaded onto flat bed trailers. They must be properly secured just like any other cargo.