I don't think there's a hard, fast etiquette on it. Just speaking for myself, I know I've forgotten to reply to a few FDKs over the years, but in general it's really important to me to let someone know how much I appreciate that they took 5 minutes out of their day to read my story and then email me. Even so, as a reader, I don't think you can go wrong with positive or constructive feedback if they don't reply. As much as we love to write, I think we'd all eventually say it's feedback that plays a part in keeping us going. If I really didn't want to know what people thought, I could just write a story and read it to myself on the computer. <g>
JD
