Singleplayer: After completing the game and feel like messing with it a bit more. On games that do not support it; injecting to memory to tweak stuff is fine.

Multiplayer: Only if everyone is aware of it (stuff like Minecraft 6b6t and CSGO HvH servers). It gives a different flavour to the typical gameplay loop.

But if the people I play with are unaware, i feel like it robs the enjoyment out of everyone.