If it is a lens with an anti reflective coating, you may want to use a glasses cleaning cloth (like one intended for anti reflective coating) if you are super paranoid about scratches. My eagletac has an anti reflective coating, but I have been unable to scratch it, and have cleaned it with less ideal things. You probably want to stay away from paper products, which can cause scratches more easily.
However, a flashlight is a tool that will eventually get scratches no matter what. Small scratches wont really make a noticeable difference in beam quality. Smudges and fingerprints, however, are sometimes noticeable when you shine the light at a white wall, but aren't all that big of a deal. They won't cause a noticeable difference in brightness.
Keep in mind this is a flashlight, not a camera lens. It doesn't need to be perfect, and you don't need to worry about getting it "soiled."