I've been a morning exerciser for 10+ years and have tried several times (unsuccessfully) to shift to afternoons. Normally, it's the LAST thing I want to do when I get off work. Plus, my mood is always better when I start the day with exercise.
Most importantly, do what works best for YOU! My husband is not a morning person, so getting up early like I do is very counter-intuitive for him.
As long as you have a designated time, that's all that matters!