Depth bias basically "nudges" every pixel forward or backwards in view-space, based on the bias value provided. So it doesn't just affect comparison with the depth buffer, it also affects the resulting depth value written to the depth buffer.

If you always want whatever you draw to end up on top, why not just disable depth testing? Or am I misunderstanding what you're attempting to accomplish?

I usually end up organising my rendering code so that it happens like this:

Render Opaque Stuff (environments, characters, etc)

Turn z-write off, z-bias on

Render depth-biased decal stuff (blob shadows/bullet holes, etc)

Turn z-write back on, z-bias off

Render 1-bit transparent stuff (foliage, etc)

Turn z-write off

Render 8-bit transparent stuff (particles, etc)

By turning off the z-write while you render your decal stuff, you can stack lots of decals without having to increment the depth bias between each layer.

Edit: Also, I almost always z-bias by creating a new projection matrix that has a minutely narrower field of view. I find it tends to succeed better across different platforms.

If your co-planar stuff uses the exact same vertices (and the same vertex shader), then there's no need for depth biasing -- just use the depth-test mode of less-than-or-equal and the latest pass will always win, or use equal for every pass after the first one. In this situation, there will never be any z-fighting.

If your decal's vertices don't line up perfectly, then yeah, you'll get z-fighting, which you can fix by z-biasing the decal towards the viewer (which modifies the decal's z values that go into the depth-test).

Alternatively you can depth-bias the entire first pass of the scene in the opposite direction, but that's a much less common choice

I prefer the first option because the right depth-bias value to use depends on the distance of the object from the viewer. The z-buffer units aren't evenly distributed (there's more precision near the camera), so a value of "-10" might mean -0.1mm when close to the camera and -1m when far from the camera