Im new so be gentle...
My question is, if I rip a CD into itunes, does some of the sound quality be lost when listening to it from itunes?
Rip it as lossless,
go into preferences, general, import settings and select "apple lossless"
When not lossless some detail is, by definition, lost, however they've also worked very hard to make it so you can't hear that difference. Depending on how much the audio is compressed, it can be very difficult to tell the difference and most (IMHO) can't between, say, a 256kb or even 192kb mp3 and a lossless. The 550s resolve a lot of detail so I'd suggest not going with the default 128kb in iTunes (if it is still 128 by default, I'm not sure it is), but you'd probably be happy with the Apple Match standard 256kb or even 192kb if you want to save hard drive/mp3 player space over lossless.
Depends what you listen to,
extremely heavy and extremely soft sounds, for me, have definite unintended distortion at peaks and busy moments...
Depends on your setup if you can hear that though.
If space is relatively unlimited or you want to archive off your CD's lossless is the way to go. but if you're just listening to stuff on the go, it's not worth the space hit you get.
You can always convert the files down*, change your iTunes cd ripping type, highlight the tracks you have in ALAC and you should be able to select "Make MP3 Version" (or similar)
There is plenty of software out there to do this for you too on all major OS's...
*you can't add what isn't there.
For almost all rips, 320kbps MP3 will be completely indistinguisable from the original ... definitely not all.
Just in case there might be a problem, and out of habit. I always rip to lossless (FLAC). I use up much more disc space, but I don't care. When I want MP3's, it's easy to convert.