The worth of making that change depends, at least in my mind, on how often you find yourself wishing you had a smaller gear(s). How long have you been riding and what kind of climbing do you do?

Like Jenn, I'm a little surprised that you LBS thinks it needs to change out the r/d for at least a slightly wider cassette. I would think it could accomodate at least a 12-27/28. Of course, that comes with a tradeoff in that you will have eveñ wider jumps between some gears.