It's not so strict, but we try most time to keep it consistent. For example, here in Buenos Aires we almost don't say the "d" at the end of the word, like in "ciudad" (city), in some pronunciation guides I saw it written with a tiny d.

If the variant get's too popular the two versions become the official spelling, for example "septiembre" and "setiembre" (September) are correct. I hate the second one and I never use it, but it's popular somewhere. After many years, sometime the old spelling disappears and is marked as archaic.