I've used a bunch of different interp settings, and currently I use 0 for everything.
I used to use .033 and .0152 and integrated them into my crosshair switcher so I always had the appropriate interp for the weapon that I was holding. After I made the interp toggle script public and Valve blocked changing interps while spawned, I went back to using 0 for projectile, and .033 for hitscan.
At first I was confused, because my Soldier shotgun was so much better than my Scout shotgun. I tried out several different settings that were supposed to make hitscan more accurate, but I got even worse results. Eventually I said "fuck it" and set everything to 0. Since I changed everything I have had almost no issues with hit detection at all. I get better hitreg with 0 for both projectile and hitscan than I ever did with any of the other settings.
This could be just me though. I have weird internet.