The major 'trick' when using the SAMPLER on the Ketron products it to use the ZOOM function (x400) to get an accurate view of the wave. Next, determine the loop (L) point (use the cursor button) such that when you do loop, you experience a smooth transition b/w the start and end of the wave. This way, you will not notice any absolute 'point'. Now for percussive instruments, this is not required; however, when you do sample these, ensure that the trigger value sits at around 7,8 (not 3,4 when using string instruments).
Hope this helps.