I think the thing is you are unlikely to piss off a million selling artist's or his label by doing a 750 selling bootleged remix. But if you nick someone elses 750 copy idea and flog that then expect him or her to be mighty miffed.

So ... you should definitely contact the artist before you released something with their sample in it, and in fact you should probably have a written contract with them before you do the release.

But if the people you are ripping off are probably going a) not care because they are too big to start with and b) going to have to spend more on lawyers than you'll make on the record (which is the case for most big labels and copyright owners) then you might do better keeping quiet and getting on with it, especially if you've got a real original work that you think the world should hear. If it's naff then best kept it to yourself! :)