I am thinking of having injectable fillers would this be advisable while I am taking 15mg of prednisone?

Thank you for the excellent question.While it is likely fine, you will need to meet with your injector and discuss your medical history in detail. The steroid treatment itself is not a contraindication, however the underlying medical condition which is requiring this treatment should be discussed with your injector first. There are a wide variety of conditions treated with steroids, some of which may lead to more concern with injections than others. 

It would depend on why you are taking Prednisolone. Prednisolone can suppress the immune response which in turn can make patients at more risk of  post treatment complications. Its always advised that you consult with a trained Doctor if you have any past medical histories or are taking any medications.