I agree with poster JSWolf. There's no point in keeping duplicate after duplicate. Once you've verified that YOUR version is fine, what's the point of keeping the original? I keep the mobi and epub versions but there is no point in keeping stuff converted from txt, lit, or other formats that I will NEVER USE AGAIN. The only exception I make are pdf files for crafts since the pdf file typically works better on the computer.
I've converted HUNDREDS of books from various formats and have yet to have one that needs to be 'reconverted.' PDF files are the only things that I've had any trouble at all with and it has ALWAYS been a problem with the original file, not the conversion.