Outside of ramen shops and very cheap places, I've never heard of restaurants that don't take reservations. If you have a specific place you want to go, reservations are always a good idea. Even if the place isn't full, you'll often get a better table if you reserve. (Of course you should be sure to call and cancel though if you can't make it.)

Especially on Thursday and Friday nights in Tokyo, good restaurants tend to be fully booked. There's nothing worse than wandering around when you're hungry on a Friday night looking for a restaurant with an empty table, and finally ending up someplace mediocre.