There are times when using antibiotics is justified, for example if you are isolating a primary culture and you can not risk a contaminated sample of tissue. Having mentioned that however, during regular cell culture the use of antibiotics should be avoided. The reasons are many but to list a few….antibiotics can result in persistent low-level contaminations and continued use can allow for resistant organisms to grow. Also antibiotics can be harmful to animal cells depending on both the concentration and media used. Additionally, and I think importantly, the use of antibiotics can mask poor aseptic technique which can become a large problem down the road. It is best to not use antibiotics and learn how to work properly and have good technique which is the best way to reduce contamination.