In reply to GaryK:
It isn't a question of safe or not. There's a spectrum of safety concerns, with greater strength and security being balanced by various other specific usage considerations.
If, as Jim mentions, you are hanging on with one hand and need to tie a knot, then it goes without saying that a knot you can't tie this way isn't going to be of any use, and so the overhand, whatever its deficits may be, is the only game in town.
If the resistance to pulling a rappel caused by knot drag is important, and/or if concern about the knot actually hanging up and preventing rappel retrieval is a problem, then the overhand knot has been tested to excel in reducing both these concerns, and hence its broad acceptance for joining rappel ropes. A second feature of the overhand knot is that it is relatively easy to untie after loading.
On the other hand, viewed out of any specific contexts, the overhand isn't a good joining knot. It rolls under loading, with cyclical loading being especially worrisome, and there are stronger alternatives that are completely stable.
My take is that if you have two hands and aren't in a desperate rush, then rappel slings, whether tape or cord, should be joined with a double fisherman's. None of the advantages of the overhand apply to knots that aren't being dragged down the rock, the slings are not destined to be untied after use, and so it is hard to see any good argument for using an otherwise inferior joining knot for that purpose.