Hi ssr, There are usually very few maintenance issues with glass – manufacturers often suggest just soap and water. Now grease is another issue, a manufacturer should answer that. What does Sicis say? Glass wont budge, if the tiles are small, but small shifts in the substrate may be translated into the grout line safely. But larger tiles are at risk even when there are small movements underneath. Sicis is a name brand, the membrane will amount to a fraction of the cost of the overall project. We suggest it won’t hurt and yes, it’s probably worth the trouble. The Manufacturer should give recommmendations and then be accountable if these instructions result in failure, so it’s probably worth talking to them about it. At any rate, if Sicis do not call for a membrane, but for availability or other reasons, you cant follow their instructions to a T, then you’ll have to make a call. We find membranes are a security measure well worth taking with no really significant downside._________________