I agree to let her ride what she wants. If she's adventurous, it makes it easier. I would be careful planning FP+ that you don't start out with the more intense rides.
As for SM, I have a story about a very adventurous little girl who struggled on that ride. My daughter wanted to ride it, so we let her. We were a little over half way through and she started crying and telling me she wanted to get off. I talked her through and was so glad when it was over. Then my daughter, tears still wet and face still red, said she wanted to do it again! She said she got upset because she didn't know what to expect and liked it now that she knew! It quickly became one of her favorite rides. On the flip side of things, she did Tower of Terror at 3 and her itty bitty self was way off the seat. She recovered and was still willing for anything but hasn't tried ToT again since that day - our last Disney trip was when she was 8. We're planning another trip for when she's 10 and she's said she's ready to try it again. (This same kid loved RRC at 8).
I do agree that It's Tough to Be a Bug deserves caution. It lasts a lot longer than a quick ride. My eldest, who has only become upset during one ride in his life, had to be carried out of that attraction due to his screaming. Granted, he was only 3, but he wasn't a kid who normally scared easily. Since he was the eldest and my youngest scares super easily, we haven't tried it again since then. (My thrill seeker - the daughter - is my middle child).
As far as the way the seats are on SM, it really depends on your child. I *think* my eldest may have not tried it until 6 because we weren't sure he could be trusted to sit nicely without an adult next to him. My middle child did do it at 5 though, as she could be trusted to sit how she should normally.