I work 10 hour days. (6am-4pm) I absolutely used it as an excuse not to work out.
IanG gave me a reality check when I posted about needing help figuring out the schedule. He basically said
I had to make time if it was important to me. He's 100% right.
With that being said, the best time to work out is anytime, as there is no wrong time to work out.
What I've figured out is that if I schedule it into my day and make it just another meeting, I'll do it. So, now my calendar has different "meetings" on it that prompt me to work out.
I prefer working out in the AM because of the energy boost it gives me, but starting my shift at 6am makes getting up at 330am pretty unrealistic. I've started changing into my workout clothes before I leave work and heading straight to the gym. The changing before leaving routine has really helped me.