I think you mean inbreeding; it all depends on the history of the lines you are dealing with. If there is a long history of good health in a line there's no reason not to inbreed. that's how showers of fancy mousie maintain the purity of their lines. If you are dealing with petstore mousies, there's no way, usually to determine the health of the mousies, and it's best to avoid it if possible. This is especially true with any newer recessive varieties like rex, hairless, satin, etc., which are often hard to get at a petstore, but they do show up from time to time.
If you get meeces from a reputable breeder who can vouch for the soundness of the lines, or the lack thereof, then you'll be informed on how to prceed in making pairings.