At least in the Pacific Northwest, a lot of the software corporate firms are much more casual than business casual. It depends, though, on the company. I usually err on the side of dressing up a bit more anyway (for example - at my corporate company, which is not a software firm, non-distressed jeans are acceptable; however, I feel more comfortable wearing a skirt or dress most days. Officially, our dress code is "business casual", but if you look at the actual items allowed, it's really more "smart casual, but there may be some occasions where more professional attire is appropriate.")
I dressed up especially during my first month on the job, even though I could see that I was in a more casual department, because I wanted to gauge what was going to be appropriate.
If you do get a job offer, it definitely isn't silly to reach out to an HR representative before the first day of work and see if they can send you the dress code blurb to make sure that you are dressed appropriately. I've had a few new employees do this before I meet with them on their first day.
I wish you very well on your search, and hope we'll be hearing about an exciting new opportunity for you soon!
