[Bill Diehl]

It depends on how far they are off and how many rpm you plan on turning.
Overall valve train geometry is everything at extreme rpm. Lifter bushings also allow you to control or adjust the oiling to the top end.

If the angle is not what the cam was ground for the cam timing is not correct nor is the valve seeing the proper lift either.

This is not to big an issue on inline valve heads but on canted heads it's very important as it will increase power and rpm substantualy.