The answer depends on the type of gauge.
But for all types of strain measurement, you need to make sure you don't have any "zero offsets" in the measurements. In other words, make sure you know what the gauges read for the condition that you want to call "zero strain". There is often some residual strain in the structure which you want to ignore, caused by things like its weight, geometrical tolerances when assembling it, thermal expansion, etc.