I think it's reasonable to clear Category. Fields that I think shouldn't be cleared are Event, Room, Date and Time. When I enter talks at a conference I typically do one room at a time and enter details as quickly as I can going down the room. So I wouldn't want things that don't typically change to be cleared.

Here's an example of a schedule I've had to fill out in the past year: http://2013.foss4g.org/conf/programme/daily/1/index.html