I use the website (simply because my computer is faster than my phone at processing; the app works well though, if that's your preference) and am making ADRs at the dot of 7am EST, at the 180-day mark. I start with the ones that are tougher to get, and then move down the list. I also try to schedule the potentially hard-to-get ADRs toward the end of the trip, as those dates will naturally have more availability.
If I am absolutely unable to get an ADR for the date/time/party size I need, I call when the phone lines open and see if something can be done.
So far, the only times I haven't been able to get an ADR via the website at 180 days, here's what happened (just to encourage you that it usually does work out in the end - YMMV):
- Arrival date was fully booked at Raglan Road. Since it's an independently-owned Disney Springs restaurant that only reserves part of its space for Disney ADRs, I just visited the restaurant's external website and made a reservation there. (This is an available workaround at a number of restaurants especially at Disney Springs. Also, at the Landry's restaurants, which are Yak & Yeti, Rainforest Cafe and T-REX, you can join their rewards club ahead of time, which entitles you to priority seating without any need for an ADR.)
- There was a website glitch one day that wouldn't allow me to make ADRs past my arrival date, even though we were staying onsite. Although at 7am there was availability at CRT for my party of 6 on my daughter's birthday, the website wouldn't let me book it, so by the time 8am rolled around, the website was showing no availability for a table of 6. I called the WDW Dining line and made a reservation for our party at "split" tables (one for 2 and one for 4) instead. On arrival, they were able to seat us together anyway. Crisis averted.
- Hoop-Dee-Doo was totally booked on our desired day, and the only available dinnertime at 'Ohana on our desired date was 2 hours earlier than we wanted. I set up requests using the touringplans.com "Reservation Finder" tool, and eventually -- weeks later, after missing a few opportunities by not clicking fast enough when the tool notified me of an opening -- was able to book Hoop-Dee-Doo, as well as get a more attractive dinnertime at 'Ohana.