Boiler is brass. From observation mineral deposits seem to accumulate most rapidly at the water/air interface on the surface of a part with evaporation taking place leaving the deposit, followed by the same event over and over at that area. Most Cremina boilers have some scale about half way up the inner surface evidence of when the seal failed and the machine sat for long enough to let the minerals build at that point. A machine just taken from use will have a light coating of mineral on the element and bottom plate, but not enough to impede function . It seems that the water has to dry out to really build up any appreciable scale. Think of your bathroom faucet.....if you have hard water the faucet will show a buildup of scale around the opening and possibly over the surface but I don't ever recall the pipe ever becoming plugged.
So from the standpoint of the machine, it would be worse in a way to empty it and let it dry on the inside over and over thus building up layers of scale (particularly if you also leave the cap off so it can dry out repeatedly). Especially if you use hard water in the boiler.