IMHO you should ALWAYS ALWAYS undercoat your models, it means that the foundation paint sticks to the model much better. GW use mould release agent that often means it's hard to apply thin paint directly onto the model. If you're after a dark look in the recesses (e.g. the scales) why not apply a wash afterwards. I always paint the scales, apply a wash, then highlight.
If you're wanting to cover a lot of models quickly, think about getting an airbrush. I undercoated my CoC with chaos black spray, then used the citadel airbrush to give them all a dark angels basecoat (saves painting each one individually)