I was thinking about this for a bit and, while I agree that the newbie may just look back on his first trees one day and decide to plant them in the ground as they are not bonsai-worthy, I advocate for working on as many trees as early on as possible.
Personally, I have devoured whatever bonsai literature I could find and (almost) made a pest of myself here with all my questions, many of them hypothetical. Absolutely nothing, however, has clarified issues as well as actually working on a tree. Did I kill trees? Hell yes. Did I butcher others? Another resounding yes. And still other trees should never even have been considered for bonsai. But all this hands-on experience brings Merrigioli's book and the advice received here into sharper focus. The mistakes made will allow me to develop trees that are actually bonsai-worthy.
For this very reason, I have a few maples that I refer to as the "front line". These are maples that I purchased at big box stores for about $15 each (it cost me more to make the grow box) and are what I experiment on. If you've ever read Of Mice and Men, I give these trees the "Lenny" treatment - I love them so hard they die. When I trunk chopped, these guys got chopped first. When I root graft, these will be the first to get the treatment. I learn on them and make the inevitable mistakes. These are not great bonsai material, but I am learning a lot and, when not stressed by it, am having fun.
It's like learning to ride a bike. At some point, you just have to give it a go and scrape up your knees and elbows.