Guys we have to make a clear destinction:
"Spanish cedar" (cedrela) doesn't have anything at all to do with "real" cedar - it just got the name because it smells slightly similar... And as has been mentioned, it is closer to mahogany, and is used a lot, both as a body wood as well as for acoustic guitar necks.
So, using that for necks shouldn't be a problem at all.
"Real" cedar (actually, there are quite a few different kinds in that family - red cedar being a very common one) is a completely different story, it is a VERY soft wood, it is used for acoustic guitar tops, but I doubt that you could use it for a neck...
So, as long as we don't know what the OP has, it's hard to give advice, because we are talking about two completely unrelated and different kinds of wood that, unfortunately, share the same name!