It's important for any project that you like the feel of the knitted fabric that you are making. The needle size on the yarn wrap is a recommendation but you can adjust that as you prefer to get a fabric (and gauge) that you like. For most projects you'll be given a gauge (stitches/inch) in order to produce the size desired. (which may be more critical for a sweater than for a scarf). You can adjust the needle size in order to obtain the sts/inch recommended. Or you can adjust the pattern to the gauge you like.
Size 7 needles are a little small for a bulky yarn but again, if you like the way the scarf is turning out, then that's just fine. Here's a chart
that shows you recommended needle sizes for a give yarn weight.