I am not sure there is any benefit to running pre-emerge checks for pre-built binary packages. It seems like it is a waste of time and checks. The packages are already built, so any check would have been done on the build system. Checks for like temp space and other do not apply as it will not be compiling, just installing. I think it could speed up time to install binary packages. Either way I think it not necessary to run for binary pre-built packages.