The rather entertaining "Box O' Truth" website did a pretty thorough test of this.
Cutting "x"s in bullets is harder than you might imagine.
It might make them slightly more effective than Ball, but not
nearly as effective as modern Jacketed Hollow Points.
The .45 ACP did not expand at all. This is probably the result of
a much thicker jacket than the 9mm.
The lead .38 Special shed its petals, but didn't expand at all.
Cutting the end off a rifle Ball cartridge will definitely make the
bullet expand or break up. Would this make it more lethal? Probably.
But not as much as a modern JSP (Jacketed Soft Point) bullet.
Reversing a [rifle] Ball bullet will cause it to expand and break up. Old
Elmer said that it killed game just fine. But Elmer didn't have access
to modern JSP ammo. If he had, I'm sure he would have preferred it.
I added the bold (and the second rifle note), as it's a critical distinction (rifle vs handgun). The only times I have seen this in hollywood is on handgun ammunition, which behaves very different from rifle ammunition.
It's not bulletproof (ha ha), but it is informative.