Always clean and grease spindle tapers and crank sockets.
Older alloy cranks like Campagnolo can distort. Use a torque wrench.
Newer cranks have stronger forged sockets.

Remove, clean, inspect, grease, and reinstall loose cranks. Don’t just tighten the bolt.

Always clean and lightly grease the axle tapers or splines before fitting cranks. This contradicts advice which is often given, but anyone suggesting that grease shouldn’t be used either doesn’t have any mechanical understanding or has a vested interest in shortening the life of your cranks. — Chris Bell, Highpath Engineering

I worked for Race Face […] and their engineers […] advised: “Grease the tapers, but make sure you only tighten the bolts once, then leave them alone.” Their tests had shown that a “dry” spindle/crank interface did not result in a consistent press-fit between the parts.

[…] we developed the new René Herse cranks, […] our engineer mounted the first test cranks without grease, and found that they had unacceptable levels of runout of the chainrings […] When the runout changed each time he mounted the cranks, we realized that the cranks were not seating uniformly on the taper.

If you grease the interface, they will slide smoothly until you stop turning the bolt when it is tight. If the interface is “dry,” the crank catches on the spindle. […] Even with the same torque, the crank will sit differently [….]
— “To grease or not to grease?”, Jan Heine, editor, Bicycle Quarterly