Dilemma

The first day at a new job always brings up the question of what do I wear?  I know what the dress code says that I can wear, however is it acceptable on the first day to wear something comfortable and casual? Of course, the other part of that question is if I wear something that is more professional, less casual, will I be over dressing?  Tomorrow is my first day at the new job.  Well, actually, it's my first day but I won't be in my home office.  I will be in Austin participating in a new employee orientation.  So I imagine I will be around the big bosses, instead of my direct supervisor. As I go over this question, this dilemma, in my mind, I'm really considering wearing slacks instead of jeans...
I think trying to figure out what is appropriate to wear the first day, when the dress code varies depending on the job and location of the job, is one of the most difficult things to deal with.