Saved lettuce seed, or left to reseed, can grow nice stuff. I have romaine growing right now. In fact I made a salad with some last night! I suppose if you have a bunch of different varieties growing together, you might have some crossing, but lettuce has a very low crossing rate. The recommended distance between varieties is not very far. Plus they have to be blooming at the same time since lettuce flowers bloom very briefly. So you really could grow a couple varieties together and not worry much about crossing.
So if you like a variety, save that seed! : )
Remy

Thanks Remy!!

Ive got lots in the same pot, so Ill just have to see whats blooming when and think about it. The romaine is in its own pot, thats kewl that it comes true.