It's hard to know what's optimal, especially as I'm only on my second little one now (DS2) and don't know how much TV has impacted DS6 or in what ways. DS2 has an expressive language delay, so I let him watch something called Baby Babble for a while. He's also watched a fair amount of PBS shows, and some movies.
I've read plenty of concerned posts elsewhere about possible negative effects of signing, usually about a baby speaking later because of comfort with the signs. We've never used signing, although DS2's speech therapist began doing some signing with him, later abandoning it since he began picking up with verbalization. My hazy memory of prior reading on signing, though, is that you shouldn't be concerned if your child becomes comfortable signing, because there's no real ill effect from it. It's still expressive language use, and your daughter can pick up vocabulary without any hindrance. If you want to encourage verbalization, I wouldn't refuse to communicate by signing, but just do it gently. It will probably sort itself out.
I definitely understand the desire to avoid too much stimulation. The worry I've always had is that my kids will become less able to concentrate on their own, without a constant barrage of signals to return to task. It's the reason I never buy any electronic early development toys that reward a button press with sounds and flashing lights, etc.; if received as a gift, I stick them in the basement and donate them later.
That said, we've probably let both children watch more TV than many parents (ETA: at least those who give a darn about their children's education), although a large amount of it is educational. I've bought the kids stuff like "Planet Earth", "WW II in HD" and "The Universe" complete set on Blu Ray, since I figure that when they feel like watching something, it's better for them than "The Cat in the Hat" or a pure entertainment TV show, or rewatching "Iron Man" for the umpteenth time. DS2 has watched some movies, mostly ones like "Fantastic Planet", "The Bear", "Dumbo", Miyazaki movies, etc. which I hope stimulate interest in certain areas for him. We've cut back a lot on TV watching in other areas-- I often insist that my wife turn off the TV when it's functioning just as background noise for her.
For some reason both children seem to have a longer-than-normal attention span, but that might be a quirk of their personalities and I'd hesitate to draw any conclusions re: their watching habits. It could well be that they'd have better focus if we hadn't taken the edge off, so to speak.
