When I do them, I do them at night to reflect on the day that's passed. Part of that is that I don't have time in the morning, and part of it is that I find it easier to relate the cards in retrospect.
A story I've told many times is when I was studying Clive Barrett's Ancient Egyptian Tarot, I got the 3S for my daily reading. The imagery in this deck is different from the RWS version as you have a naked couple on the card. The woman is sitting on the bed and the man's back is to her as he stands in the doorway. I think it's thundering outside. I couldn't see how my day was reflected in that card until I remembered that a co-worker had tried to argue a point with me and I quite literally turned away from him cause I wasn't going to continue down that path.
Had I pulled that card for the day to come (whether in the evening before or the morning of), I would've been looking for heartache, pain, discord, and would've ended the day thinking, "Hmm.... THAT didn't happen...." But looking back on the day, I was able to make the connection.
Rodney