I like to post excerpts from my novels as I write the rough draft. It's fun and I feel encouraged to know that someone besides me may be reading and enjoying what I've written.
You might notice that I don't post more than two or three chapters from a given book or I post only a paragraph or two from a handful of chapters. There is a reason for this.
Most publishers now consider that any piece of fiction posted on a blog or "public" networking site has used up its first publication right. (As of this point, password protected forums do not count as "public". Facebook, MySpace, etc do.) This is especially true to publishers of short fiction. Which is why I do not post even excerpts of shorts online.
Now. Some people will say posting any excerpt is not a good idea. They may be right. I figure I've not put enough out there for anyone to steal. And, I only post early drafts, not polished versions. Given my writing method the bits posted here are unlikely to be exactly like the final (hopefully) published version.
I point this out because some of us writer-types don't research our markets before we start writing. *raises hand* That would be me. I had several short stories written and posted on Facebook before I realized I was killing my chances for getting them published. I won't be making that mistake again.
And, just in case it isn't obvious, publishers nearly always want first publication rights. (Editors for anthologies are a bit different. They are usually looking for already pubbed work. But, they tend to not pay as well.) It is difficult for them to try and sell anything that is already available for free. So, nothing pubbed on a blog is eligible for "first publication."
Will any of this stop me from posting snippets from my rough drafts? Probably not. At least not until I get an agent or publisher who tells me different.
Should you be wary of posting snippets on your blog? That depends on what your goals are. And just exactly how paranoid you are. ;)