There are no risks, but it really screws up with the hormones. They want you to wait due to scheduling issues (for example, so you don't get your period during the weekend) and also so that your menstrual hormones are out (and you're not putting extra hormones in and screwin' it all up).
Starting it earlier will not help you out any...it is just best to wait. Since you're waiting on your period, it sounds like you are very close to starting so best to just do so.
However, there are no risks involved in starting early.