I assume that by "multigrade filters" you are referring to the set provided by Ilford, which I believe are made of a form of acetate. Other filters such as the older Polycontrast filters, or the equally vintage Durst filters, are made of different materials, and so may be able to survive a wide assortment of cleaning agents.
For the Ilford set, here's a quick outline of a safe cleaning procedure: 1. Blow away loose dust with a squeeze-type blower. 2. Brush any remaining loose dirt with a soft brush. 3. Dip a small rolled up piece of paper towel in a mild sudsy water solution, and gently apply to filter surface. A very small amount of the soap solution will do. 4. Dry with a paper towel. 5. Blow dry with the squeeze-type blower.
Do not use the following, because they will eat into the dyes of the filter or into the plastic itself: acetone, alcohol, PEC 12, most lens cleaners, naptha/Ronsonol, Windex or other ammonia-containing cleaning solutions, household bleach, and other common cleaners.
Again this applies specifically to Ilford's multigrade filters.