Well, yes you do and no you don't.
If you have the BOV after the MAF (like in a suck-through setup), and it vents to the atmosphere, then the engine will assume that the air read by the meter is still going through the engine, and you'll get a momentary rich mixture.
If the BOV is before the MAF (like what you might find in a blow-through setup), then the vented air will never be read, and engine operation will be normal.
On a speed-density system, it isn't an issue at all.
But, if you are venting air read by the MAF, then you should route it back into the inlet side of the turbo to prevent a bad reading... Sure, your car may run this way, but I'm a firm believer in doing it right the first time... Why engineer something to be wrong?