The only way to tell is to buy a new ski, ski it for a few days, then get a tune from a reputable shop (not one that rails your skis). If you notice them being easier to engage and release, with no loss in stability, as if you just bought new tires for your high performance car, then you benefited from the tune. If no difference, then yeah, either you had good skis to start with or your skills may not be at a point where a tune matters all that much..
Actually, I always use the same shop. One time, it was terrible and my skis came back railed. They fixed it, but that was the only time I experienced a railed ski. Now, I always use a flat edge to check if the ski bases are good after a tune. I just never felt the need to do it upon a new purchase.
Yeah, my skills are pretty average. I'm definitely not a racer or even an expert skier. I'm a recreational skier who manages about 40 days a season - weekends mostly. I was just wondering if this is something that most people notice with new skis or just the (I don't even know an appropriate term) ... extreme experts?