Diamonds,
Congrats.
Yes, you can go without an InfoPass appointment. I got my stamping done two days ago (I had an appt). I assume you would be going to the JFK building, Boston. When I was over there, I heard an employee telling an applicant that walk-ins are permitted but the person should be at the JFK building as early as at 5 AM. They permit around 450 walk-ins a day. Then you will have wait for 3-4 hours because they consider the appointment cases first and then come to the walk-ins. So, you will have to be prepared to wait. The earlier you get there, you stand a better chance of getting your work done sooner. Also, to keep in mind, that in the room E160 (where the stamping is done) area there are two lines leading to the room (big waiting area with counters).
One on the left is the longer one (without appointments) and on the right is for appointment cases.
Take the necessary documents. The actual stamping takes around just 5 minutes.
Hope this helps.
Good luck.