SIgning your work on the back is fine @honeycombwinnie-- quite a few artists do. It seems more common with paintings on canvas than pieces on paper which are more commonly with a pencil signature on the front, at the bottom of the image. If you sign your work on the front, be sure to leave what we call a "framing margin" (1-2" or 3-5cm) so your signature doesn't get hidden by a frame or mat.