My dogs have always worn both a collar and a harnesse. Tags are on the collar and a leash and/or tether clipped onto the harness. I have never clipped a leash or a tether onto a collar.
I think a lot depends on the individual dog. There is a big difference between a Bull breed and a Chihuahua! If your dog is very strong, you may have more control using both with a double ended lead. If your dog is biddable, a harness could be enough for general purposes. Do be careful that the harness fits correctly though, and doesn't interfere with shoulder movement, or could be backed out of. Personally, I only use a harness for tracking. I use fabric half-check collars for walking and training, but for safety would always have flat leather collars on them when travelling - all with tags of course. I think the majority of training classes would recommend using a collar once your dog had moved out of the introductory class.
Generally used with collars and dog leashes, I always choose to fit a set of styles so that it looks more coordinated.
