Read this from sample craze about sampling, there are some good tips on when to sample mono or stereo and just good tips in general.
http://www.samplecraze.com/page.php?xPa ... Guide.html
Basically, you can always make something stereo if you need to, by assigning it to a 2nd pad, then panning. The only reason youd want to sample it in stereo to begin with is if it contained an effect or desired result from the get go-similar to what samplecraze is sayin. Like if there is a vocal, or some crazy organ, panning effect,etc.
Just because your end result is stereo and you might apply some effects and pan shit, doesnt mean the sample has to be in stereo to begin with. If we are talkin the raw peices to a track, then I keep everything mono, except the main sample (but im sampling from vinyl).
Also another tip to sampling in stereo is that if the sample is right, you can erase or clean out the sample: for example: if the drums are panned to the right and vocals to the left, you can just keep the left channel and get a cleaner vocal sample.....or whatever it may be. Sometimes you can cancel out vocals too, by mixing the same channel...... I keep the main sample from vinyl in stereo because of this reason. I have more options with it later.
Finally, I do notice that its difficult to mix drums in with some giant stereo samples; Its harder to match the drums so they dont sound too flat.......but this is part of good mixing. So just because your samples are mono, doesnt mean that the end result will stay mono, sometimes its neccesary to apply effects and do some panning to get things to sit right.
peace