Check OSHA as well.

If your penetrations impinge on the clear access and egress of the walkway or grating, you might have clearance issues that are more important than toe plates. Toe plates are there for several purposes....to prevent items from being incidentally or accidentally pushed off the edge, creating danger below; to prevent feet from doing similar or getting caught through wedging into tight spaces, and to define the geometry of a walkway or accessway. Since toe plates are typically required to be 4 inches high, I would set a limit to the penetration size, that if exceeded, a toe plate would be required...such as if the penetration is greater than 4" in any direction, a toe plate is required.

Is the deflection of the grating compromised by the penetration? If so, you might require reinforcement in addition to a toe plate, though they could be combined.

". . .you might require reinforcement in addition to a toe plate, though they could be combined".

Combining them is an elegant solution. Welding every other bar to a slice of schedule 40 pipe [flush at the bottom of the grating, extends 4" above grating] will use the strength of the pipe to carry the grating loads around the hole you cut in the grating. If you don't use this solution, you typically have to call back the ironworkers to add junior members to support the grating at the hole that the pipe folks just cut.