Don't leave anything floating, period. Tie to a potential, usually the supply rails, but it can be any node, like to the substrate.
During the fabrication process the DRIE step (and others possibly) leave charge on anything floating. Once the insulator (SiO2 or interlevel dielectric) is deposited on top and around everything, then the charge will be trapped there , forever, or worse, it'll leak away over the years. This modulates conductivity of electrical devices nearby (think EEPROM-like behavior). The charge build-up during fabrication might also be high enough to rupture the insulation betwen the dummy and something else, and then you'll have a short between node, through the dummy. Think "ANTENNA RULE" .
Layout people are aware of this (most of the time?).