Well I did once reuse needles multiple times but in more recent years I would change the needle after every injection (as per the manufacturers instructions), but it's entirely up to the individual and questions like this always draws a mixed response.
Today's needles are designed for single use. Reusing a needle could lead to an increased risk of infection at the injection site, also can make injections more painful as the modern needles blunt quite quickly thus are less efficient at puncturing the skin. Personally I would never use a modern needle more than once. Xx