The difference between FLAC and MP3 is nothing like as pronounced as some want us to believe. I can discern a difference with some recordings but not for a majority and not reliably. And in no cases does good quality MP3 sound any worse than FLAC for normal listening in my view. If you need to concentrate hard and still cannot identify differences between FLAC and MP3 with total reliability (in my experience that is everyone I know if they're honest) then that tells its own story. I use FLAC now as memory is cheap and I tell myself it is better but I am playing the same psychological mind game as sighted comparisons and think MP3 is one of the most unfairly maligned technologies in audio.
I have 4 folders on my HDD that are isolated lossy tracks that have issues i kept in case i do a true DBT on foobar.