The only way to find out, really, is to render. The Bias value seems to be in centimetres, like all other D|S distance measurements. So the edge of e.g. a sleeve curling in to just a few millimetres from the arm inside will be affected if you leave the Bias at what I think is its default value for a new light, 1cm. It’s the usual tradeoff — go for good shadow fidelity and accuracy, or a not-too-long render time, you have to strike a balance between them. FWIW, my usual starting point for experimenting with Bias values is 0.1 or 0.2.