When I write a philosophy paper, should I be concerned with developing a personal style? Or are philosophy papers best written in a manner similar to physics lab reports or mathematical proofs--that is, in a technical, impersonal way.
Neither. Assuming by "philosophy paper" you mean student essay, then what you need to be doing is evaluating arguments, as carefully and as honestly and as rigorously as you can. You must aim for maximum explicitness, maximum clarity, maximum organization of your thoughts. But you are writing ordered English prose, not lab notes or a mathematical proof. "Personal style" will look after itself, and shouldn't be your conscious concern. (It is always a pleasant surprise to me, e.g. when I have to mark a stack of undergraduate dissertations, how -- despite the fact that students have gone through the same teaching treadmill and drilled by weekly one-to-one essay tutorials -- distinct voices will always come through.)