Lots of guys train AM and PM, and some in between as well. It really all depends on how complex your workout schedule is. How "hardcore" are you willing to go, or how much time do you have to work with. If you are going pro and are preparing for a contest, your ass will be trying to work every muscle you have and you will probably be spending a good part of your day in the gym.
The average guy in the gym usually works his primary muscles, if you start to focus on muscles that you don't normally work, you can find yourself working out all day.
Bottom line, you just have to make sure you give your body enough time to recover, that's the most important thing to consider when making a program. You also have to recognize what secondary muscles you are working when hitting the primary ones. It's a simple matter of math. IMHO!
/V