Just in case no one else pointed this out:
I can't speak for the JWs because I don't know how they organize their missionary efforts. But LDS missionaries don't serve in their hometowns, and they only spend so much time in any given town before they're transfered to another area. So it's possible that you get knocks on your door after declining to talk to them just because they haven't personally met you or knocked on your door.
Since I'm LDS and know what our missionaries go through I like to be polite to JWs. I don't think being insulting is ever the answer. I know they only bother us because they sincerely believe and want a chance to share that.
Actually had some LDS missionaries knock on our door the other day (we're new in town so they didn't know we lived where we do). We gave them some water before they went on their way. It gets awfully hot walking around in suits and ties.
I think (in general
) our LDS missionaries will do their best not to bother you if you ask. But I also know they get a lot of doors slammed in their faces, harassment, encounters with people who hate them and think all LDS people are going to hell, etc. so a polite "no thank you" means the world to them.
ETA: It's not about bothering people who are busy to tell them they are wrong. Some have suggested that it's perfectly acceptable to be rude to people who come door to door...but if the tables were turned and someone treated you like crap because you wanted to tell them about breastfeeding, or no circ, or vaccines, or [insert thing you sincerely believe in and care deeply about] you'd probably be offended. Like I said, I can't speak for how the JWs do it, but LDS missionaries give up 1.5-2 years of their life, on their own dime to volunteer full time (and it's not all going door to door, they spend a lot of time just doing service for people). They don't do it to annoy you, or to bash you, or to interrupt you're busy life. They do it to share something that is important to them and has made them happy. I think acting like they don't deserve to be treated with as much respect as anyone else because you happen to be in the middle of something when they come by is a mean attitude. Some posters seem to think missionaries deserve to be treated like dirt and I think that's ridiculous. No one deserves to be treated like dirt for simply asking if you'd like to talk to them about something very dear to their hearts. Like I said, if someone was nasty to you just for trying to talk to them about your beliefs/thoughts/opinions/whatever I imagine you'd be pretty ticked off.