Keep in mind, using a grease injector needle you are actually going THROUGH the seal, therefore, making a hole in the seal. I would think that popping the seal off so you can thoroughly clean out the old grease and possible rust build up, not to mention being able to visually inspect the bearing, is a better idea. Especially as easy as it is to pop off the bearing seals on both sides. Just my $.02.
--Steve (O.C.)