In the mire of timing constraint

What is very important? I woudl recommend entering constraints up front as much as possible. Besides making it easier to meet timing, I think the overall flow is easier. If you do it on the back-end, you need to re-analyze the logic, make sure it really is a multicycle of false path, and then enter the constraint. Instead, while you're designing and explicitly making logic that handles transfers as a multicycle of false path manner, that's when it's easiest to create the constraint. I'm not saying some of them get missed, but I wouldn't save it all for the back-end.