There have been various opinions expressed about this very issue on this board, and the long and the short of it is, some feel that glycerin is appropriate for tack cleaning, while others do not. Personally, I was taught (via Pony Club, which is gospel to many, heresy to some I suppose) that using glycerin is the final step of a three part process to take care of leather (clean, condition, and preserve). Since the glycerin is designed to seal and protect the leather, it is not really appropriate for cleaning, after all, what's the point of sealing up the leather before you condition/oil it? However, as I have previously noted, other people feel that it is perfectly appropriate for cleaning tack, I'm just not one of them
My saddle is about 30 years old... here's how I clean it most of the time (I clean bridles the same way.) Get your sponge fairly damp... not dripping, but not completely squeezed out either. Rub your tack with the damp sponge (it should be wet enough to leave a little water behind - you are removing yesterday's layer of glycerin and today's layer of dirt and sweat). Rinse out the sponge well and ring it out well so that it is just a little damp. Rub the glycerin bar with it. Rub your tack with the sponge so that you put a thin layer of glycerin on. This seals and protects your tack. Tomorrow after you ride you rinse it off and repeat. If your tack gets really dirty you clean it with Castile, oil if necessary and add a layer of glycerin.