I think you may find different answers to this one (and there are a number of studies out there on this), but it will really come down to your own natural body clock, your time management, and what works the best for you. Are you an early bird or a night owl? You may find you will stick to a more consistent exercise routine if you listen to your body.
If you are a morning person–good for you! Working out in the morning may both help you be consistent in your program (because you will be less likely to make an excuse in the morning, compared to later in the day when things come up), and it could help you sleep better. If you exercise too close to bedtime, it could interfere with your sleep. That being said, if you find you can get in a workout in the evening and it works for you, then go ahead!
Bottom line is whatever works for you!