ESD when working in case?

Unlikely.

Unless you wear silk socks and rub around on a wool carpet, ESD is just not a big deal in my experience. I've built hundreds of computers and upgraded far more and never worn a strap -- although I am careful to ground myself by touching grounded objects.

Yep. There shouldn't be any problems. I'd advise you do generally not do builds above carpet, but otherwise don't worry about it.