I do, probably because I'm a very visual writer. I like to see the scenes as mini-movies in my head, and that's hard to do when your characters keep changing appearances. (Mine change races, sexes, and ages on me all the time.) I rarely actually describe a character in the story, but it's good to have a mental image.

Mentally casting an actor in the role kind of solidifies the character for me, especially if the voice is also right.