I would make different decals for different surfaces. It's not that much work, and can provide a lot of visual fidelity. Wood would have a splintered silhouette, plaster/concrete/stone would have a chunky edge, metal would have a scalloped edge, etc. I like how DICE did their decals for Battlefield 3, with parallax mapping…
Yeah, what Eric says. Most games that have decal bullet textures have a material flag for every object to define what surface type it is, and the relevant effect (and sound) is used. Some games even have a map so you can define multiple surface types per object, but that is a bit less common.