There is continual progress on standardization of methodology, but it takes a while. People have vested interests in using their protocols, and there often valid disagreements about what is actually the best protocol.
Nevertheless, these kinds of standardization documents are important. But only as long as deviations from these standards are not discounted when justified theoretically.
Pre-experiment mouse handling procedures as outlined in section M.1.2 and M.2.4 of the Mouse Handling Procedure Standards Body Publication of 2014
Which document could have any number of sections detailing the finer points variable control for the relevant experiment.