I ride a cross bike (Kona Jake the Snake) with a low gear of 36X25. At the end of May I rode three mountain passes with 12,545 ft of climbing in 133 mi. Personally I found the climbing easier than the 20 knot head winds and though a 27 on the back would have been nice for about ten of those miles, it was hardly neccessary. I have a 10 sp cassette and my riding partners had a 9 and a 7 respectively. One more gear seemed to make a small difference in the rolling sections and 3 more I think made a big difference.
If you are moderately fit and your bike without water weights less than 25 lbs IMO don't change a thing. It gets to be kind of fun grinding away after a while and highway grades are easier than you think.
We are planning a 29,000+ gain ride for next year. If I don't get a new set of wheels I doubt I'll go throught the hassle of changing my gearing.