I started emailing only the professors who I thought knew me well enough to agree to writing a lor. After 4 saying no and 2 saying yes....I just emailed every professor that I have ever taken a class with. With the second method I got 16 no's (or no response), 1 maybe and 1 yes. Am I wishing I had gone to a smaller university? or, wishing I had been one of those kids who *never* put their hand down during class....asking questions no matter how simple the material? Perhaps...but, can I change that? no.
Moral of the story, email everyone and get another letter. You never know how the letter will sound (when I met with the professor who finally agreed to write lor #3, he told me that he loves helping people get into grad/professional schools...that he admires peoples intentions). If you find a nice professor, they may write a nice letter whether they know you well or not.