I think the 15 minutes is an informal rule of thumb. After all by the time you have texted/called the customer and then made the delivery it could well be as close to the top of the hour as makes most difference. Of course, it always help to explain to the customer if they appear flustered or not ready that you are a bit early.
I have suggested in the past that the customer should be offered an option for them to be happy to accept early delivery or not which would then appear on the delivery advice notes. Some are always very happy to, a few, a very few are adamantly against (eg if they work from home and tied up in a meeting or appointment).