If it's the jagged splinter scheme that you're going to paint, I think it would be better to mask since the edges are straight lines and angles. Personally, I don't think I have that steady a hand to make perfect lines and angles on large scales. But that's just me -- you may fare better. In my pre-airbrush days, I would mask when the camo called for straight edges. Sometimes the paint jobs were perfect with no paint seeping under the edges of the masks, but other times they were messy. I guess I just didn't know the right techniques back then (even now I'm still learning). So for you, my advice is just be sure to burnish the edges of the mask well to prevent paint from bleeding under, as well as laying thin coats of paint so as not to saturate the edges. Better yet, seal the mask edges with a brushing of clear coat or with Future. Of course, let the clear coat cure properly (at least 24 hours) before you begin the actual camo painting.
For curvy, blotch-style camo like that on WWII British aircraft, I would normally draw the camo outlines on the model with a pencil just to guide me and I'll do it freehand. For mottles, it depends on what type of mottle it is. Your suggestion would result in solid-colored mottles, if that's what you're after. I've already done a Hasegawa 1/72 FW190D-9 and the box art as well as the painting diagram showed a soft-edged mottle on the sides (read: spray-painted) which I could only achieve with an airbrush. Hope this helps.