Some observations from our experience this year. My wife and I had never been to Cinque Terre. We stayed 3 nights in Manarola (Da Paulin - Conchiglia apt). Arrived Thu 5 Sep (train from Siena) and left Sun 8 Sep (train to Milan).
I'll preface by saying that I don't do well in crowds or long lines, becoming quite grouchy. Thus I strive to create itineraries that minimize encounters with crowded conditions.
We were very satisfied with this early Sep visit to Cinque Terre, despite some crowded moments.
Staying in Manarola, we visited all five of the towns, including going to Vernazza and Corniglia three times each. We would gladly stay in Manarola again.
The crowds were absolutely awful on only two occasions. The worst was a short visit to Riomaggiore on a Saturday. It was truly a mosh pit at the waterfront as both trains and ferries disgorged hordes of visitors. The other time was when we changed trains at La Spezia upon arrival on Thursday, boarding a very crowded milk run train with our luggage; however, it might have been especially crowded because a train (ours) had arrived at La Spezia 30 minutes late.
During the day (regardless of day of week) in both Manarola and Veranzza there were times when tour groups partially blocked the narrow waterfront pedestrian sidewalks/trails as the folks stopped to take photos, but it was not unbearable.
For two of our three visits to Corniglia it was easy to catch the shuttle from the train station right after disembarking from the train. Saturday early afternoon (NOT morning) was the exception: most people (including us) chose to walk up the steps to the town as the wait for the shuttle was quite long. (We had also arrived at Corniglia about 8:15am that same Saturday morning to hike; at that time there was no crowd disembarking and it was thus easy to catch the shuttle.)
We never stood in line for a train ticket. I used the Trentitalia mobile app (or accessed their web site via PC) to purchase tickets for our arrival and departure from the Cinque Terre. Also, for our two full days I purchased hiking / train Treno Cards. Though we only hiked one day, the cards saved us money and stress on both days as we rode the train multiple times.
One evening we attended the opera in Vernazza. Nice experience, and the room was only half-full.
We did one hike, from Corniglia to Vernazza. We started on Saturday morning about 8:30am, and encountered few people during the 90 minutes until the very end, when folks started departing Vernazza to hike the opposite direction to Cornigila.
The local folks' attitude in general seemed almost indifferent to us as tourists, being neither negative nor particularly positive. For example the two times we bought a few groceries at the small co-op in Manarola, the store folks didn't smile but were very helpful - which was how I noticed they were also treating most local customers. And a couple of elderly women we encountered outside our apartment didn't offer greetings until I did; one of them even smiled in return. It reminded me of the local service during a late summer trip we took to Dubrovnik two years earlier - perhaps this late in the season folks are just kind of tired due to the long hours of the tourist season.
This was our first trip, and was wonderful. We are definitely happy to have seen the Cinque Terre during the busy summer season. If I visit again, I would likely choose to arrive during a different season in order to see the region under different weather conditions. BTW, one night during our September stay there was an hours-long thunderstorm; this caused us to delay our hiking one day (from Friday to Saturday.)