Should I let my baby suck her thumb?

I first let her have a pacifier at 5 weeks, after feeding was well established. She would only take it at certain times, mostly at night but as soon as she found her thumb the pacifier was out of the question. She sucks her thumb a lot, even during the middle of the night. It soothes her, calms her, etc. I'm not sure if it's true about causing a lisp, etc. What are your experiences with this? I think it would be really hard to break her of this.