I was under ther impression that all once fired cases had to be trimmed to 1.750, is this correct or not ?
No this is not correct.
223/5.56 brass has a trim to length of 1.750 and a max length of 1.760. Anything in between is fine as far as proper fit goes. For ultimate accuracy all cases should be trimmed to the same length whatever that may be, as long as it's between 1.750-1.760.