Yes, minidisc's lossy but, unless you're gonna a/b it with DAT then it's unlikely you'd notice - especially with the latest versions of ATRAC. The sample rate is 44.1, so no worries there.
As BBS says, it's not a "pro" format, but it still sounds good. The trick, if you're really concerned, is to monitor the output of the MD deck while mixing, as opposed to the ouput of your DAW or whatever. That way, what you hear is what you'll get.
It's so easy to get hung up on pristine sounds - I've had stuff released that's been recorded (not very carefully) on stereo cassette (the only versions which had the right "vibe", oddly enough...). MD isn't DAT, but it's a great format and easy to work with. Just be sure to make some backup mixes.