What will happen if you leave the main free is the boat will start sailing around on the rode. In very light air it doesn't seem to happen but minute you turn your back the wind will come up and....Try it and play around with it and see what happens. I'm not saying make the sheet tight. Just so it's not flopping around. Another thing I do when I'm camping is tie a line around the bar on the anchor. Make the loop small enough so it won't come off. I take the end of this line up to the beach. A retrieval-safety line if you will. At low tide I can anchor out in the deeper water and as the tide comes in I can pull the retrieval line so I don't have to swim out to the boat. This line can be tied to something like a tree on the beach so even if the anchor was to pull the boat is still attached to land.

Have Fun
Mike