At 100000 miles, how necessary is it to have a valve adjustment done, as the owner's manual indicates?

My 2008 Honda Civic needs an oil change, and the indicator says that it is also due for a 100,000 mile valve adjustment. The Honda dealer service tech says that it will cost about $150. Is this a good price for this, or would I get a better deal at any competent auto shop? The service tech tells me that in a worst case scenario, if this is not done, the valves can stick and cause the head gasket to need replacement. Is this a realistically valid concern, or am I better off going by the old saying, "if it isn't broke, then don't fix it?" the car runs fine, but it will need tires soon. What should be a priority, the tires or the valve adjustment?