Its perfectly fine, as long as the person who owns that base or picture is ok with you using it. Everyone is different and how you learn to draw can be very different than the way other people did. I myself started out the same way, tracing/using bases. I believe it helped me learn how to shape things and it was excellent practice.
As an original basemaker, I provide my resources for people to do what they like with them, tracing them blowing them up whatever, however I just ask that if you used it in any way shape or form you link back to me as your resource. Pretty much all basemakers ask the same thing.