For me a 5mm difference sure is noticeable. Several decades back when I was in my late 30's the body started telling me I need to make an adjustment. Tried different length stems and between 2 stems of different lengths of 10mm difference one was too long and another felt a little too short. Bought one 5mm in-between the two and it was like magic. There used to be more stems available in 5mm length increments but not too many now. Been using the same seat to bar measurement ever since and can tell when it's not the same. That's just me though so not the same for everyone.