The usual recommendation that I've seen in my little circle of friends is to keep the feet close together. If the feet are too far apart, sometimes uke lands on nage's leg, potentially causing injury to nage's knee via pressure from the outside of the leg while the leg is being used as a support.
I almost injured my know once doing the technique with feet apart. It was a big guy and one leg took all the weight. Don't have the same problem with feet together. Feet together increases strength.