If the joint at the juncture between horizontal and vertical surfaces opens, it becomes an area where mold and/or mildew are likely to proliferate. And as most of us are now aware, mold/mildew is one of the latest and hottest litigation specialties.
In most cases, mildew-resistant flexible caulk will prevent such problems. Under "materials" in the Movement Joints section of their 2002 Handbook, the Tile Council of America (TCA) says silicone sealant may be used. The silicone sealant - used for sealing joints in ceramic showers and around tubs, sinks and plumbing fixtures - must be single-component mildew resistant, which means it contains fungicide.