Roundup will not mess up the soil. Many spray in the spring and the summer then plant ginseng seeds in the fall. I have not done it but many do. I would not hesitate to do it if I were planting in an area with many weeds. Beware thou, if you have other desirable plants in the area Roundup will kill them as BCastle had stated.
You probably need to be aware that Roundup will do a fine job on grasses and many broadleaf weeds, but it may not do a good job on woody plants such as young Maples, Hickorys, Oaks, and heavier brush that is often found in woodland that is growing back from being cut some years before. You may have to do some grubbing here. Also be very cautious about becoming exposed to Roundup with your bare skin. It can cause some bad side effects. The ingredients become inert after contacting the soil and should not cause any problems with new planted seed.
Roundup only works on green leaves. It is pricey, so don't waste your money now. I often spray small saplings that I want to kill off (Sassafras particularly) For smaller sapplings which are rooted somewhat more shallowly, like maples, I just pull them up by hand. I spray rose bushes and then pull them after they die off, or in the spring, I just use a grubbing type hoe and pull them and make sure the roots are high and dry, then spray any new shoots that come up.
Things need to be green and growing for Round Up to do a good job. We are at the time of the year that plants are going dormant and you probably won't get as good a response as if you wait until next Spring or early Summer to spray. You will probably be able to do your best grubbing on young saplings and other stubborn woody type plants while things are dormant and more open to work on. I think it is a little easier to plan beds at this time as well. Good luck.