Don't count on TV's hypnotic powers to keep your crawler or toddler safe. Your child still needs supervision, even if she seems entranced by a particular show. Make sure the TV's in a child-safe room, that all stairs are blocked off, and that you can see your baby from where you will be.
The amount of television your baby watches, and the content of the programs, shouldn't be an issue until the end of the first year or even later. Until about 9 or 10 months, your baby sees TV programs as nothing more than a blur of flashing lights and colors. With most shows, even children's shows, the images change too often to permit your baby to focus. (Some infants do find this blur somewhat hypnotic, but most find it boring.) Even after your baby develops some interest in TV, it probably won't sustain her interest for very long. After all, your child still has an extremely short attention span.
It's not that watching TV is such a bad thing to do. But your baby could be doing so many other things, things she won't be doing while watching TV. So when TV does become a viable option, try to limit viewing time. Just as important (if not more important), limit your baby's viewing choices. In general, stick with public television, shows on Nickelodeon or The Learning Channel, and well-chosen videotapes. Almost all of these choices permit commercial-free viewing of non-violent, somewhat educational programming.