The only time you are going to know it's moved out of square, unless it stalls, is when the gantry is back at the home switches. So with this in mind I'm going with one switch and will create a datum point each side to enable checking with a DTI as suggested. If upon the first attempt at homing the machine I can see that Mach3 moves it only a very small amount I may then consider using two switches but the last thing I want is for the homing procedure to gradually 'loosen' the gantry structure.