Its called slight d.i.
Yes, it is true that forcing Marth to pivot grab is good. But most CGs in melee require some sort of d.i. in order for a combo opportunity to present itself.
Marth CGs Fox: If you d.i. left or right, Marth can fair, nair, tipper, re-grab. He can do a million and one things. Neutral d.i. isn't a whole lot better. But at least if he wants to tipper you he has to walk a bit and force it. D.i.ing during CGs is like handing them combos. Mind you, its good to mix it up sometimes - but on the whole, neutral d.i. followed by really good combo d.i. when they hit you is the best approach.
fox CGs Fox/Falco: If you d.i. left or right, they have the opportunity to re-grab, usmash, nair, bair....
but the problem is, they can do most of this with neutral d.i. anyway. The only advantage not d.i.ing brings to this situation is the fact that your trajectory becomes more random.
Sometimes if Fox nairs you or usmashes you during neutral d.i., you fly behind him or something. This could be due to a mistimed move or whatever, by my point still stands. Neutral d.i. reduces the chance of you getting super comboed. *imo*
What people normally do is:
Fox uthrow-> Falco d.i.s right -> Fox nair -> Falco "noooooo!!" d.i.'s towards Fox so he doesn't die -> Fox responds with a super duper star fox combo.
There is one recurring point I should mention when it comes to CGs.
The solution to the majority of them is usually to neutral d.i. and shine or jump. Marth @ 20% and above, Fox @ 55% and above, etc etc.