AAC to FLAC makes no sense. Lossy -> lossless makes no sense. You're simply increasing size with no benefit and only negatives.
ALAC -> FLAC...maybe a benefit if one of your system has issues with ALAC. FLAC is more universal.
AIFF -> FLAC...you can get smaller sizes and save space while having identical copy of the music. That's a positive.
I keep music in both FLAC (highest compression) and WAV. Both have a purpose.