One thing about converting an mp3 into a wav...if you're doing it to import it to FL that's one thing, but you're gonna be working with a low quality sample most of the time (unless the mp3 you convert is 220+ quality). When you convert a mp3 into wav, you're basically making a very high quality copy of a low quality original, and that lost quality can't be restored just by converting it to wav.
When i first started making beats, I heard people say you need to sample from vinyl or at least burn straight into wav from cd and keep it that way, and I was skeptical, but it definitely makes a huge difference in the quality of your final product. My advice is just to record a song or rip a song straight into a 2160k wav file (highest quality possible in sound forge) and use that.