A good way to determine the point of transparency of lossy encoding for yourself, is to ABX test on your own equipment, with files you've converted yourself. A good way to do this is with Foobar2000's ABX plugin, which lets you compare back and forth and on whole tracks or short snippets if you want.

In my experience, headphones always yield the best results, and surprisingly it doesn't matter if I use the stock earbuds from my phone or a nice set of AKG over-ear headphones. It's not a matter of absolute sound quality, just the fact that you cut out room interactions and get the sound straight to your ears makes a big difference.

MP3 has some built-in flaws that no encoder can completely cover up, short sharp sounds like castanets really expose the pre-echo, harpsichord shows similar issues. It also has a tendency to make cymbals sound "washy" or "underwater", which all lossy codecs do to some degree, but MP3 is especially bad.

Still, at 192kbps I have to really focus to hear it in normal listening, but it's more or less always there even at 320kbps in problem tracks, if I really focus in on short sections. It just sounds subtly "off". But I hope no one actually listens to music like that, in short repeated sub-1 second sections to narrow in on a specific castanet snap ;-)

As for more modern codecs like Opus and AAC, it's generally completely transparent for me at 128kbps, and that's with a bit of playing it safe, I'm pretty sure I could drop Opus down to 96kbps. Modern codecs are really impressive.

I keep my music library in FLAC, both because I know it's CD quality and because it's an archive. I want to be able to convert the tracks to any new codec that may come along, if I need to.

My library is 280GB currently, and storage is cheap :-)