I think it's almost always better to draw the original graph dotted. Drawing the original graph allows you personally to see the relation between the original graph and the transformation better, and is especially important when drawing transformations like y^2=f(x) or y=[f(x)]^2, since the relation between the original curve and the new curve for 0 < y < 1 must be clear to the marker. On the other hand, if you're asked to draw a transformation such as f(1/x) or e^f(x), the original curve isn't as important, and you can probably leave it out (as long as you still label some critical points).
