If you want to make a few bars of soap that are exceptionally moisturizing for the Winter season, and plan to use them within a few months, it is perfectly acceptable to superfat as much as 15-20%. Keep in mind that you have to allow a cure time of 4-6 weeks after making the soap before you can use it, so, if you need extra moisturizing soap for Winter, get busy!!
Butters are commonly used as superfatting agents. I personally use Shea butter. I only superfat at 5% year round because I like tons of lather and a nice hard bar of soap. If extra moisturizing is more important to you and you can live with the decrease in lather and a softer bar of soap, superfatting is the way to go.