I've only ever shot a slug (Brenneke rifled 1oz slugs) through a cylinder bore, but I've got five interchangeable choke tubes for my Benelli. I was wondering if slugs can safely be shot through any choke other than cylinder, and if so, what would be the tightest choke that would be appropriate?

You can shoot Fosters through any standard choke. Brennekes too. The "rifling" on Fosters and Brennekes is to minimize contact area with the bore and allow swaging, it does not provide spin. While you can shoot these through a full choke, word is best accuracy will be through a modified choke or looser.

Garmentless is correct. I have had best results with the Winchester SuperX Fosters through an improved cyl. I can consistently get 5-shots to cloverleaf at 50yds with open sights. Brennekes shoot almost as well. Pricey though. Once you find the right ammo/choke combo, smoothbores can shoot almost as well as rifled barrels.

I stand corrected. Brenneke does not recommend chokes tighter than improved cyl.

www.brennekeusa.com/web/text/4_2_text.html

IIRC improved is the way to go.  Cylinder bore will not allow the rifling on the slug to impart much spin.  

Just check with the manufacturer.  I have yet to by a box of slugs that didn't have the recommended choke listed on the box.  I saw an article once somewhere that defined what the optimum difference between bore diameter and slug diameter was.  I forget now.  Anyway, I get yahoos around here wanting to shoot slugs through full choke bird and trap barrels all the time, makes me cringe and my teeth hurt.