I use WAV for all critical listening. However, source quality is key. I have CD's in which the recording is simply so poor that I can be just as happy with a classic 128kbps MP3, while I find others yield more detail in full-blown WAV format.
This thread is completely valid, but so many people on this board, including seasoned members, quickly forget the basics when it comes to audio. EVERYONE HEARS DIFFERENTLY!
That said, anyone entering this thread to "inform" the OP that what he is hearing is all in his mind or that he can't possibly hear a difference should be ashamed of themselves. This equates to suggesting that you have "golden ears" while the OP can't possibly hear as well as you do.
If you don't hear a difference between WAV and FLAC or WAV and 320kbps MP3's then simply state that, but don't tell the OP what he should or should not do, hear or otherwise. It's all in presentation, and if you post in a belligerent manner in which you sound as if you're suggesting you know it all then obviously people are going to become defensive and even ignore you and your advice.
Personally, I find that storage-space is so easy to come by these days that I don't know why I would absolutely have to encode anything. I don't keep everything in WAV because some music simply isn't recorded well enough for it to matter, and there is a lot of music which I rarely listen to because it's not my favorite so I don't find the need to keep it in WAV, etc. If I'm putting music onto a media player then I do become concerned, but I keep the stuff I want to sound the best in WAV format while the rest gets 320kbps MP3 treatment or often 128kbps because systems outside my headphone rig aren't going to reveal anything important a lot of the time, anyway.
Happy listening!