It depends on the piece.
If I look at the log and can picture the piece I go for the final cut right away, if it does not come to me then I will block it in. I always tell people to trust yourself and don't over think your cuts as far as is it the right cut to make. If you can picture what you want in the log then just cut it. The only time to over think is for a safety issue, other than that trust yourself, its art there is no wrong or right just what you see. Some day just take the saw and start cutting with no idea of what your making and see what you get, a picture of something will come to you as you go. Its fun there are no rules of what it should look like, its just you and your imagination. Most of all enjoy carving, don't worry about what everyone thinks. In time, the ability to copy images and carve fast will come, don't rush it new people, when it happens then people expect a work of art everytime you lift your saw. I do this full time for a living and am sponsored by a saw company, and have been carving for 10 years. The most fun I have now is carving by myself without an order, no pressure. So enjoy and have fun newbies, and don't over think, you have plenty of time for that.