it depends on what exactly you are looking to accomplish with the tuck.
basically, how tucked is tucked?

I will say it is far easier to do with everything out of the car.

I've done some that were as simple as running the harness through the intake manifold. and others that involved tearing the whole harness apart to re-route, lengthen, shorten, re wrap to get the desired results.

personally, I think it is more work than it is worth. But if you are big into the look, then I won't stop ya.
My only advise is to make sure everything is in good shape before you get into the tuck. You don't want to fight the tuck to replace a sensor or something simple like that.