Like anything else, there are plenty of opinions on how a tree should be planted. Soil, climate, time of season and type of tree can alter the method of planting slightly. Comparing information you have heard against solid sources, like universities, extension offices and forestry services can help determine what the recommended method is for your area.
Trees need proper moisture, loose soil to establish roots, and oxygen. It must be planted appropriate to the type of soil and water drainage of that area. Clay soil should be left loose or punctured, not smooth. In some cases you may need to dig deeper than necessary and add amended top soil. Planting a tree with the flair slightly higher than soil level, keeps it from sitting in water for slow draining areas, but allows it depth to build up wind resistance as it roots in. Ideally a tree should show the flair of the trunk above the surrounding soil level, with roots 1-2 inches below the soil surface. Back fill the soil around it to keep the tree straight, but do not pack it down too much depriving it of much needed oxygen. Water immediately after planting to help settle the surrounding soil and allow the planting area to be saturated deeply. Top dress with mulch to help the area retain the moisture you add when watering, but keep the mulch from piling up against the trunk where it would encourage softened bark and insects to congregate and invade the tree bark.
