A flight instructor stopped me on the ramp the other day when I was doing my preflight. He told me he noticed me put the fuel that I sumped back into the fuel tank. He said you should not do this. My philosophy has always been as long as the sump is clear of particles or moisture then I can put the sample back into the fuel tank. If it has any contaminents then I dump it on the ground. How do you guys feel about this and what do you teach your students?