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The 5p test is a good one, but the precise answer will be on the BMW TIS DVD. Mine is a 2004 copy so doesn't have the answer for E9x cars, but for example for a E46 330d it says the front discs need replacing when the thickness of the disc is 23.4mm. This needs to be measured excluding the lip mentioned above (which will likely be almost the same thickness as a "new disc" as it doesn't wear on the pad) - hence a small lip is acceptable as long as the majority of the disc is still thick enough. The disc should not be unduly scored or uneven, but if it isn't and its thick enough then no need to change it. If you can't get hold of TIS just ask the BMW dealer what the change thickness is.
Bear in mind however that if it is near the thickness limit, the dealer may opt to recommend changing the disc as it will perhaps be under tolerance during the life of the new set of pads - and everytime you change a disc you need new pads, so not changing could be a false economy. I read somewhere that M5s have pads and discs changed at the same time because of this (i.e. disc life is less than 2x pad life - a bit silly in design terms if you ask me though).
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