Some of us have tested FLAC format. For two-channel, it works well. ALAC might work too, but I haven't tested them on the cars in Tesla's demo inventory yet (others on the forum may have, and my memory sucks today). The majority of your CDs are very likely two-channel. You'd know it for sure if they weren't: they wouldn't play on cheap equipment like a discman.
If you choose to rip into either format, it's very likely to play well in Model S without a problem. If it doesn't, you have "gold masters" to encode later to lossy formats that are known to work. And that can be automated so you don't have to swap discs for days. If you start ripping now, you might be done by the time your model S is delivered.
If your hearing is less-than-a-perfectionsist's, like mine, you probably can choose a lossy format. Unless you have a reference studio close at hand, you would have to be really good to pick out the lossless from lossy-format audio playback. I personally can't. There will be others who say they can. It's a personal choice. MP3 or AAC at 320 kbps or better, will likely make you very happy. The iTunes Match idea from pilotSteve intrigues me... It sounds like a GREAT way to fetch high-bit versions of music without breaking open a single CD case.