Only the temperature of the Luminase enzyme will affect the results and this should be at room temperature when running the assay.
Cell activity, however, will vary based on the temperature - at lower temperatures, cellular metabolism and reproduction are slowed therefore ATP levels will be reduced. This is why it's recommended that samples be tested immediately at ambient temperature to ensure you're getting the most representative results.
If samples were stored cold, allow them to return to ambient temperature before running the test protocol.