Soybean weed management is critical in the first 6 weeks after emergence. This is when the plant can suffer yield loss by weed competition. The weeds are also competing for nutrients and water. On the other side, if you have a soybean variety that will make up for early season competition such as a forage type, then leaving the weeds for a couple of weeks may reduce deer damage. If you time your gyphosate to just prior to canopy closure, then you can get by with one spraying. This can be argued that you are promoting weed resistance by doing so. Weeds should be sprayed according to size and not crop age. Most herbicides are labeled on weeds when they are 2-4" in height. With a product like Glyphosate, it only kills what is up the day you spray. It has spoiled most farmers by not having to scout for weed species anymore. We have been putting pressure on weed resistance for many years. Fortunately, there are still several good tank mix partners and preemerge chemicals still available for soybean.