you just gotta listen to see if it makes a difference... for some samples it sounds cool.... for others its shitty...

and as far as bpm-matching.... if your going down a lot in bpms... like from 100 to 70.... expect it to sound a little weird.... kinda metallic... due to the fact that your stretching the sample pretty hard.... sometimes it gives a good gritty effect.... sometimes it sounds plain shitty.....