Stepped Stencil Limit

I've used stencils with 1 and 2 mil steps successfully, but recently I've seen a problem with a 3 mil step (8 mil foil stepped down over most of the printed area to a 5 mil thickness). The problem I'm seeing is that the stencil tends to warp around apertures, particularly in areas like fine pitch QFPs, where there are long apertures close together, and the remaining material is a thin strip. This warp makes the stencil act thicker and allow more paste to be deposited, leading to bridges in the fine pitch QFPs. Is there a rule of thumb regarding the amount of step that can be etched into a stencil?