A snake on 12"? Anything big enough to clog that is going to take a heck of stiff snake, which will take Governor Swartzenegger to push it, unless a truck can get down to the outlet, which is not always the case. On the other hand, it will be almost impossible to clog, provided your inlets are protected and your pipes can self-scour.
If the pipe is fed by standard rdl's, everything in the 12" will be under 12" anyway, so you are OK. If fed by grated inlets, everything in the 12" will be under 12" anyway, so you are OK. If fed by an open endwall, you'll need a trash rack on the inlet endwall, then everything in the 12" will be under 12" anyway, so you are OK.
Throw an O&M note on the plans requiring vaccuum cleaning of the inlets when sediment reaches 6" deep, if you have them. If you have an NPDES permit, O&M will be required.
If the pipes can't be laid at a slope to proved scouring velocity of 2.5 fps, you need some way to flush the pipes, and a cleanout is probably the cheapest. Careful though, 12" PVC fittings are expensive. An inlet, if less than, say 5 feet deep, might just be cheaper.
Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve