That is a good question, it is pretty much unavoidable to do any restoration without the use of reproduction parts. Some of us are very fortunate that these parts have been made available, and most likely saved a few cars from not being restored. The quality varies on some of these parts and some are very noticeable that the are reproduction, and some are very good! I have a several parts on my 62 Biscayne that are repro, and some are very good, and some I feel are very poor. My options are limited because some of these NOS parts just do not exist, however I have had received a deduction for authenticity on one particular part, but if I did use the original I would have gotten a deduction for condition, so I had a decision to make. So to answer your question it only becomes a problem when it is visibly noticeable You should investigate the quality of the part and also see if it is made by more than one vendor. Also be careful there are some vendors who sell "seconds" on E-bay, I would suggest that you purchase directly from established catalog vendors, it will be easier to solve any problems.
Good luck
John Mahoney
VCCA Assistant Judging Chair
