In my experience, 1/4" plywood is too thin and flexible on it's own. If laminated to 1"+ of foam, it has sufficient rigidity.
I have successfully used 1/2" plywood with risers spaced as far as 18" apart with no problems. Watch the spacing of risers on turnback curves. It is easy for the risers to get spread further than intended on large curves due to the plywood not crossing the grid at right angles.
If you are going to be cutting your own plywood, why not cut your own grid (or even better, L girder, which requires less accuracy on the cuts) pieces as well.
just my thoughts and experiences, yours may differ
Fred W
If using 1/2" plywood for subroadbed, I would feel secure about risers spaced as far apart as 18". Five-eights would be good to near 22", with three-quarter inch good to 26".
If you want to conserve costs, and can build 12"-18" spans without incurring more costs for more risers to support it, use 3/8-1/2". Some guys won't even think about using anything less than 3/4" ply for roadbed, but I feel that it is unnecessary. Supported luan would be fine, just more labour intensive to support it to keep deflection under gravity to a minimum.
The thing to remember is transitions into grades; that is where luan or masonite or 1/4" MDF will do a good job for you. Fasten the lowest portion flush with the grade, and then allow the material to curve upward naturally to meet your intended grade. Fashion supports below that vertical curve, and you will enjoy smooth running. You could switch to 1/2" or even 5/8" plywood for the planar subroadbed.
I've used 1/4" luan laminated to 1/4" foam on 12" centers for years with absolutely no issues at all. I do have a climate controlled area and I don't crawl on my benchwork. I also model in N scale and all of my scenery is lightweight. To give you an idea of how it holds up, this is a section of the staging area from my last layout that was up and in use for 3 years. As you can see, even though I have it leaning against the garage wall, there's no warpage at all.
http://i83.photobucket.com/albums/j319/pcarrell/N%20Scale%20Trains/Salvage/Picture030.jpg
http://i83.photobucket.com/albums/j319/pcarrell/N%20Scale%20Trains/Salvage/Picture026.jpg
I think many people over engineer their benchwork. It's very important to have solid, stable benchwork, but it doesn't need to be able to withstand a tornado or something. The exception would be if you need to climb on it for some reason, but not many of us do that really.
If you were to spread out your centers, then you'd have to use thicker wood.