I usually run 5k if only because that is what is usually available around here. I also run half marathons and 8k. I haven't run a 10K in 20 years — but that's because I either have a conflict or other races that I am doing when the 10k races are scheduled.
I think that ultimately, I would like to train for general running fitness — not necessarily to peak for a specific race. I just want to run well and compete when I enter a race from 5k to half marathon. I might even run an occasional race on the track if the opportunity arises.