as long as you don't cover up the vents I don't think it is a problem, off cause if the system starts shuting down while you're playing it's too hot, but otherwise I would think you're just fine! 🙂
when I play gridwars for 3 hours straight or starcraft all day, they both run through Rosetta so my MacBook gets pretty hot as well, and it hasn't crashed on me once yet... 😉
but I'm no heat expert so I'm not 100% sure that I'm right...