My advice after ordering every iPhone since the 3G on launch day....
There’s no guarantee of anything, when their servers are under so much load the website will be crashing and timing out frequently.
The app is the easiest way of ordering but you will most likely find some quirks with it due to the hammering it will be getting.
In all honesty I’ve always managed to get one for launch day. However nothing is a given and we have no idea of stock levels, will the high price help out by putting some people off? Etc etc.
It’s mostly down to luck I’m afraid.
I tend to have the website and the app on the go and generally find nothing comes up until 8.04 ish. Last year it was nearly 8.10 by the time it came to life!!