Regardless of your playing style, there are some things you'll want to know anout the mechanics of training. First, it takes the same amount of time to train one character up a certain number of levels as it does to train all your current party members up the same number of levels. As long as you're training one character you might as well train them all.
The other important thing to know is the amount of time it takes to train. Training up always ends at 9am and takes one week per level plus the time needed to advance your clock to 9am the following morning. Thus, if one trains up several levels, the first level will take at least 7 days 15 hours and subsequent levels will take 8 days.
This gives a particularly useful strategy to those of us who wish to train. By only training one level at a time we can spend the hours between 9am and 6pm helping natives, saving the world, and shopping for cool stuff.