Potential Versus Real

In this case, we are talking about heat. We are debating whether we should fill the boiler system even if we don't have the rust inhibitor. The fellow who said he would mix up the 'stuff' for us over the weekend, has apparently gone out of town until Monday.
We still have to clean the dusty radiators in the rest of the house so they don't wind up with a baked-on finish once we do turn on the heat, so we have at least one more day to ponder.
Pro: We can test the system for leaks (and have heat) without worrying about getting rust inhibitor all over the bathroom.
Con: We will have to pour the rust inhibitor into the system through a little hole in the towel warmer and risk getting rust inhibitor all over the bathroom.
The weather may decide for us, but so far it doesn't look too bad.