When I was a kid, I could understand all the words in G & S, cold. And unmiked. Now I can’t even follow the TV without subtitles.
I’ve seen follow-along libretti for grand opera, but my dad never bothered. He just liked the sound of it.
Someone (on Tumbler, probably) contributed an account of their grandmother seeing Hamilton. A long-time musical fan, she found herself in NYC, stood in the Cancellations line, and scored a front-row balcony seat for $100.
Grandma loved the show. (Don’t know if she’d heard the recording.) She especially complimented the production for “enunciation.” That is, she could understand every word. Nowadays, all the singers are miked–so you should be able to hear everything. With first-rate tech & first-rate performers, that is…
(Years ago, a local company did a low-budget Shakespeare series. One guy I know played in Macbeth. Afterwards, he asked if i could understand him. I could. He was an Equity member who’d gotten a dispensation to appear. Professionals, in plays musical & not, work to make sure you can hear all the words.)
My wife is a theater buff and has been all her life. She went to college at NYU so she could be close to Broadway. We’ve been married for going on 15 years now, but she has subscribed to the Paper Mill much longer than that, probably since the 1980s. I just get to go along for the ride. 
“I am Figaro, factotum of the city,” sung eight times in a row sounds a lot more impressive in Italian.
But la-la-la-la-la-laaaaaah is the same in any language.
I love musical theatre. I definitely prefer to be familiar with the cast recording(s) before I go to see a show, yes. I’ve never worried about “spoilers” and since I am especially interested in clever lyrics (Sweeney Todd is one of my favorites), it really helps.
I’ve tried to do the same to get into opera, but I still struggle to enjoy it.
Generally for new musicals I always prefer to go in cold as I think that will give me fairer view on whether I actually like the show or am simply responding to music I already know. For older popular shows that have been around for a while, I have likely already heard the music.
Though I did break my own rule for **Hamilton **and finally listened to the cast album on Spotify. I tried to hold off until I could see the show but since that likely won’t be for a looooooong while (unless I win the lottery!) I had to give in and listen to it as that may be the only way I get to experience it. (Now I can’t get it out of my head! Dammit it’s so good!)
Not intentionally, but I’ve never been to a musical where I wasn’t already familiar with the songs.
Of course, I never went to anything new, either–it takes forever for stuff to come around here. And half the time it was school stuff and I was part of the performance, or had been in the past. So I kinda had to know it.
Wait–take that back. I just remembered I went into Into The Woods blind. And I think it counts as a musical?
I’m probably sticking my foot in, but have you done it going backwards, that is, the lighter, more musical-like material first? The “beer, beer, beer, beer!” kind of opera, such as Die Fliedermaus or The Merry Widow 
Into the Woods is a musical, yes. That’s one where I was very familiar with the music for years before finally seeing a production of it (first at Shakespeare in the Park and then last year’s Off-Broadway revival.)
I kind of love that description.
(But because I’m me, I’m also going to point out that it’s Fledermaus, not Fliedermaus. Fliedermaus would be “lilac mouse”. Which is probably very cute.)
Is that the one where everyone’s wearing Liederhosen?
I have to admit, I did spend years wondering why those things were called song pants.