theres no simple answer, and yes you when you normalize in post, you also normalize the noise floor of the weakest link in the chain.
i use my setup (cmr>ad2k), peaking at -25 dB and do it all in post, and it works out fine as the noise floor of the ad2k is like -115db.
for your minidisc, the noise floor is probably -80 to -100 dB, so if you are post-normalizing more than 18dB you'll probably increase the audible noise, depending on the environment