Newbie question

Hi there

Is a placement of a log (as a part of my roots) permanent?

No, nothing is permanent except your heart log. You can place, destroy, move or whatever any of your roots. Use them for permanent travel routes, for temporary scaffolding, or for mining out a cave and picking them up behind you when you leave. It's all fine.
Same goes for your tree as a whole. If you don't like how it's turning out, feel free to chop the whole thing down (except the heart) and start over. Syhd actually did this a few times on the old server.

Great. My design process is very iterative so I will probably redo a lot.

Too bad I've already died twice (damn creepers) so I am def. not doing this hardcore style.