In my opinion the best time to workout will be in the mornings. It is good in the morning because it bascially is a great jump start to your day, in a lot of ways. When you workout in the morning you are setting up your body to burn calories for the rest of the day. When you exercise you are burning calories during and even after your workout is over. Exercise that increases your heart rate significantly, heavy aerobic exercise, can increase your metabolism. It is this increase in metabolism that gives the body the energy boost to keep burning calories through out he day. If you exercise in the evening, try to fit it in by 6pm. This will allow your body to keep burning calories for some time before you go to bed. Also, late workouts can interfere with sleep schedule because after working out you have so much energy it might be hard to fall asleep! However, if you are just doing some weight workout or yoga type stretching this might not interfere with your sleep patterns because it does not increase your heart rate as much.
So try to get that workout done with in the morning and you will feel better throughout the day and keep burning some extra calories!!!!