Tom is right that adding circuitry always degrades the signal. Thus a unit that includes the control will sound wors than a unit without the control if they are otherwise identical. HOWEVER, if you are asking whether USING the control will degrade the signal, then probably not much. I have to agree with Tom that those things need to be forgotten on any gear over $1000 as that price level means you should be fixing the room placement instead.
Use of Balance Control feature
I have noticed that some pre-amps and integrateds omit the balance control. Do many people out there use their balance control under normal circumstances? Does biasing sound towards one side degrade 3D soundstage, or have any other negative effect?......Sam