The SA gig IS part of the interview process, essentially. But it's important to look where the power is.
The OP has an offer. He's deciding whether to take it. It's his/her own, additional investigation into the firm, it's culture, it's people. It will NOT be any kind of formal interviews. It will be him/her visiting with the people on a more personal level to gauge whether the firm is a good fit. When YOU have the power, you no longer need to wear a suit if no one else at the firm wears a suit, IMO.
Once you're in the SA program, the power shifts back again. You're competing for a permanent offer, and you're back in interview/impress mode.