Wood would be better then nothing (leaving it exposed), but metal is better.
Although, I don't think this applies to 99.9% of installations that are in industrial locations which are required by law to use locking metal enclosures for any power equipment.
If this was a home project and you had to or wanted to use wood, I would line it with fire rated sheetrock on the inside.
Actually, I just wouldn't use wood, but I'm not you.