I can only speak for myself, but I usually apply any fuel, oil and or water stains at the same time as I do other pin-washes for weathering and before I spray any dust glazes. I usually make the contrast between the center, more or less, clean area of the spill with the edges of the spill and don't worry overmuch about how the center may contrast with the rest of the just dusty areas of the model.
I make all such spills overall matt unless I have some very specific final effect in mind. I try to convey any residual "dampness" through the use of color (generally darker tones contrasted with lighter tones gets that across). For darker "oily" tones of the base color, I use a Payne's gray or Indigo "filter" over the base color to darken it as much as I wish, usually fading this towards the "purer" base coat in the center with the "oiler" look to the sides and the edges in a darker tone of the weathering earth color with final dusty colored outside edges.
I do spills in layers usually starting with the outer light dusty colors (often white and raw umber), fading them to the center, followed by the darker earthy tones (more raw umber), again fading to the center, and finally by any oily colors (Payne's gray or indigo) faded to the center with the center close to the base color. This leaves more defined edges to the colors towards the outsides of each blending together towards the center with the center more or less clean looking.
For water spills (radiators, etc), I omit the oily grays and just go with earth tones.
Of course, the shape and direction of the spill flows in the direction of gravity which is not necessarily vertical down the sides but is also effected by the contours of the vehicle. Puddled areas on horizontal surfaces are done the same way with cleaner appearing center areas.
Usually, I apply any pigments after my final overspray of Dull Coat and just as I'm ready to put the model on the base. This prevents any damage to the pigment finishes from excessive handling. However, I don't often use pigments for creating an overall dusty look (unless I'm going for a very dusty, dirty look with almost opaque dusty areas maybe with very heavy streaking). For light overall dust, I usually use a glaze airbrushed on. I usually allow this to go over any spills (again, unless I have some other specific outcome in mind).
These are just my USUAL approaches, though, which I vary depending on my ultimate desired finished look.
Finally, this is just how I usually do it. Other techniques or sequences can probably be just as valid and effective.
