I've never heard NOT to use cedar mulch. There's something about wood chips leeching out nitrogen from your soil as it breaks down, something to that effect, but since dahlias don't want much N anyway, I'd say go for it.

Other things I've used: compost, shredded leaves, straw (messy, takes a long time to break down,) shredded alfalfa hay. My only thought on cedar mulch was that its coarser texture might give earwigs and other thugs better hiding places, but then so does a pile of straw, hay, or leaves! I say try it. Any mulch will be better than no mulch if you have unrelenting heat without rain.

Wood chips only leach out the nitrogen if they are mixed into the soil. They are fine to use as mulch and will definitely help to keep the ground damp and cool. I put several inches of wood chips around my plants and water less and I'm in Mesa, AZ with a full sun garden.

Mulch Away and Happy Gardening,
Pam

try cocoa mulch if you can find it; it is high in phosphate and serves as a good humus source when it breaks down (but in hotter climes does tend to get some mold on it and clump but the mold doesn't seem to do harm and when I use it the dahlias are spectacular! also the roses and hollyhocks and peonies!
PS not a good winter mulch but I take my tubers up in October anyway.Oh and cedar does leach out the Nitrates first but as it degrades it adds potash.