Join Date: Nov 2000
Location: New Orleans, LA
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Generally it is recommended to avoid the extreme combinations - big front/largest rear, small front/smallest rear. On 9sp some even recommend avoiding two, ie, big front/largest rear, 2nd largest rear. I generally abide by this, but if I need one more gear on a short climb I will go to the big/big without hesitation since it is quicker and easier than shifting the front.
Cogs, especially the 1-2 you use most, do wear out in a 2,000-4,000 miles or so though some people report getting many more than that regardless. Although cross chaining can accelerate wear, you would have to do it a lot for it to really make a difference. If, for instance, you ride your small front/small rear all the time, I'm sure you would experience significantly accelerated wear. This is probably the worst combination since small cogs wear faster (fewer teeth=more contacts per tooth per mile) and it seems like the smaller diameters would exacerbate the angle of the chain.
If it ain't broke, mess with it anyway!