I recently had to deal with this same question, the thing that I run into (as a beginner sampler) is that I feel like I sample too much, in other words, my ending product sounds more like a remix than something totally new. Ive noticed that some cats can take the most popular song out and flip it to the point that its nothing like the song that they sampled from. Thats what I'm in the process of doing now, trying to really get into the sample far enuff that I will still come out with something dope but unrecognisable at the same time. I have started breaking my samples down to the actual beat or hits which is helping me a lot in terms of sampling but being unique with it. I've been making beats for a while and have no problems doing all orignal stuff but like classic said, you can do both and thats my target. Sampling hits is cool but sampling entire beat phrases ain't IMO.
Sampling vocals would be the exception for me, i'll sapmle a vocal phase and add an effect to it. Not really modifying the vocal as much as making it sound different. I don't sample much more than a line or 2 and if I do more, i break it up. 1 line here, 1 line there....but thats just my personal opinion on that. I suggest that you try it and see how you feel with the end result, if you have any doubts, don't do it, you have to be confident in your work or nobody else will be. At the same time, you dont want to end up with a remix either unless that was your intention.
MOF