J421,
For your casual flying 5%, 10%, or 15% should be fine. What you should use is pretty much a personal preference (unless you live at a high altitude, then you need a higher percentage to stay even). The higher your percentage of nitro the more power you are going to have. The down side is more power equals more heat. But with anything below 15% you will be just fine. Experiment around with the different fuels and see which one you prefer to fly with. Try to find some people at your flying field and explain to them what you are doing. See if you can get a tank of fuel from them to see the differences. This might be a way to avoid having to buy a full gallon of fuel just to try it out.

Hope this helps.