Are Metal Speaker Grilles an Issue?

I'm asking this almost rhetorically since I see these on high end speakers like Martin Logans, but I'm designing a home theater setup from scratch, and I'm going to custom CNC some aluminum speaker grilles for this setup. 

As such, before I go through the process I just figured I'd ask my fellow audiophiles their feelings on metal speaker grilles.  These won't be sitting directly atop the speaker, more like a few inches away from the speaker and are more of an aesthetic piece (and to some degree, a quasi security feature against children!).  

But I've got a CNC machine so I can make them to any spec.  Anything I should be mindful of when milling them so that I'm not absolutely destroying the sound quality of the speakers behind them?  I'm willing to deal with a little bit of loss but don't want to overlook something really obvious that I coudl have designed around.

And before anyone asks, no, I'm not thinking about using a different material.  The material is metal, the question is how best to use it.