Also depends on how fancy and realistic you want the model to be in the game. 50 extra polys is not that much for a few buttons, but having each key on a keyboard would be pretty extensive when using it with 100s of other entities.
When looking at a box with a texture on it, the player might think: "look at that box with a texture on it."
If it is a keyboard, try extruding groups of keys (Like the Alphabet keys, F1-F12 keys, Num pad keys, etc.) instead of each button.
Quote: "So like you said it where you might going to use it."
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Aesthetics and performance in mind.