Definitely less shielding. I would go for 95% freehand, with the shields just used to define a few edges here and there on your final layer. Ideally you want the first layer just to define the area you want your flames to be, the middle layer to give some shape and direction, and only have the real detail in the top layer, finishing off with some hot spots.
I actually find fire looks more natural without the use of any shields at all, but everyone has their own style and way of doing it, and it depends on the look you are going for. I can't remember the link but mitch (the airbrush tutor) has a killer tutorial for fire.