That depends. A good quality Y should not have much voltage drop. Of course everything will have a voltage drop if enough current passes through it. Two servos with low current demand may result in no voltage drop at all through the Y and this is perfectly acceptable. Start using more powerful servos though that draw more current, put them on an aeorbatic plane with large surfaces and fly the thing hard and you end up with a lot of current requirement that could reveal voltage drop accross any weak points.

For most planes and flying though a Y-harness is perfectly fine for things like aileron servos.