Vienna: when should we purchase tickets in advance?

On Sunday mornings in Vienna there will be a number of churches singing glorious classical masses. You will see handbills plastered around town during the week. The church where the famous choirboys sing, the Hofburgkapelle (chapel of the Hofburg Imperial Palace) requires tickets, but I think all of the others are free. You are welcome to attend even if you are not Catholic.

Buying tickets in advance avoids standing in line at the ticket counter, i.e. you can proceed right away to the entrance.
Time slot tickets are available at certain places, but are not really needed.

Here you can find musical performances which do not belong to the top tier venues like Staatsoper, Konzerthaus, Musikverein:
https://www.classictic.com/en/sitesearch/?search=Wien

Came here to ask this!

I'm visiting Vienna after Rome, and for Rome advance time slot tickets are required and sell out early (such as the Vatican Museum, the Coliseum and Forum, and the Borghese. And I find this annoying because I would prefer to remain flexible in when I visit sites and the amount of time I can spend at each (without having to hurry in order not to miss a timeslot of the next site.)

So to confirm, advance tickets are NOT necessary for Vienna!? That is great news if it is the case!

The only thing that sells out is the Opera. Otherwise, you can avoid waiting in line for Schönbrunn if you buy your tickets in advance, but no attraction “sells out.”