Yes, when David & Maddie got together on Moonlighting it was the end of the show. But there were a lot of other factors there--the 2 of them never really got along, she had just gone thru a tough pregnancy, etc. And the show was on it's last legs anyway.
NCIS is stronger than ever, if any show could put a couple together, it's the one. I'm always in the minority with that opinion, I know. But don't a lot of us find our mate thru work? I know of at least 3 couples, one being my brother. And if shows don't want to do it, then stop with the sexual tension stuff. Just have the characters be friends/co-workers and that's all. West Wing did it very successfully for years--Toby and AJ were never more than friends. They did let Donna and Josh get together, but only after they knew the show was going to end. If they want Tony and Ziva be a couple, then they should do it.