My wife and I have been using Derby Line since we converted from the J1/J2 to the
TN back in 2006. We have never had any problems with processing there. It's always gone smoothly. We're both meteorologists, which matches the TN listing perfectly, so that might be part of our success. I'd be processing there again this year, except my usual ride up from Connecticut can't do it this year, so I'm processing at Ottawa International Airport instead.
As for the time you're going, I've always chosen to go on a weekday, and during normal business hours. That way, if there are any problems, you know that your employer is likely to be there, and can be contacted by the Inspector for any questions they might have.