For interviews, I would take out all facial piercings, except for small ear studs. If you have more than one piercing per ear, take out any additional ones.
Once you have started your program, I think it's more acceptable to wear multiple piercings per ear and probably a small nose stud. Bigger earrings (i.e. small dangling ones) are probably okay in most situations. However, this mostly applies to your time in class or in lab ( but not necessarily when running participants).
Once you start clinical work, you should probably go back to being more reserved, i.e. take out nose studs as well as multiple earrings. Small, dangling earrings are probably okay depending on what population you work with.