Like most who have voted, I use as much of my original parts as possible and buy repro or NOS parts if the original part aren't up too scratch. I think to get the best results, using all original restored pieces won't get that.
You have to remember the factory wasn't perfect making these cars and some interior pieces certainly lacked quailty from the factory, I myself see no point in restoring a car to "that" standard. I'd rather use new parts to getting a better fit and finish. I'm not overly picky in terms of what parts are used as long as it's the best possible part out there for my car. If quailty lacks in the repro part I'd spend time restoring the original. Fenders for example, you could spend hours upon hours repairing & restoring an original fender and at the end of it not get the best results you could have. You can go out and buy repro fender, cut and weld it in and be done with better results.
Most of the repro stuff we've come across was utter rubbish. "Made" in America but sourced via Taiwan or China.
Some are passable and some are good. It really depends on what parts and for what car.
Good topic by the way.