Hi Andrew
I think it can be a matter of personal preference. I know at least one skilled modeller who never undercoats. But, for my part I always, always undercoat using a spray can of White undercoat, bought from the local Games Workshop store. It's not cheap but it works for me - not least in sharpening up the detail, but I also find it helps get an evenness of tone.
As to decals, I think a good application of Johnson's Clear (Klir) is an absolutely essential base.
Best wishes
Nigel