Are gating controls useful in large multicolor panels to give accuracy to the gate assignment. FMOs are particularly useful in 10+ color assays and when the expression of a positive population is poorly defined from the negative population.
FMOs, or Fluorescence Minus One controls, can be extremely helpful, especially when your target antigen is expressed at low levels, or you are concerned with the amount of spectral spillover from other fluors in a large panel. As the name suggests, an FMO consists of the full panel of antibodies while leaving out the target antibody of interest. In some instances, an isotype control can be used in place of the test antibody, which can provide a more complete picture of background signal, from both non-specific binding and fluorescent spillover.
For additional information on FMOs and compensation, read through this publication by Mario Roederer.