Some landscapers out here will lay down plastic before covering it with gravel. It prevents weeds quite well. But you have to be really careful with how your land is pitched, and water runoff. Out here we get no rain for weeks. Then we can get a half inch in 20 minutes when a super storm rolls through during Monsoon Season. (Now, until around the second week in September). If the ground can't absorb it, (which it has enough of a hard time doing), the plastic can really cause problems if you don't have enough pitch to carry the rain water away.
What you're talking about is called weed-block and it's different from what we're talking about. Weed-block is a semi-permeable material that people use underneath layers of mulch or rock to prevent future weed growth. It works fairly well.
What TAB and I were talking about is a method of killing every plant and seed that may lie dormant in a patch of soil prior to planting. Basically, you raze everything and dig a small trench around the area. Wet the area and then cover it with thick, clear plastic for the 6 hottest weeks of the year and then fill the trenches in with dirt to weight down the plastic. The heat created by this will kill everything up to about 18" beneath the surface, including weed seeds that can lay dormant for up to 5 years under the right conditions.
After that process is finished, it's advisable to put down a layer of weed-block before you landscape. It will keep you mostly weed free for quite a while, though once the layers of mulch start to decompose, they will form a layer of soil above the weed block in which newly spread weed seeds can germinate.