Temp fluctuation in cellar

People say it does, but it's really unknown for beer. The specific reason that it makes a difference for wine is that the temperature variance produces a pressure variance, which moves gas and vapor back and forth through the cork, typically accelerating ullage. That doesn't apply to (almost all) beer.

The variation you describe is not very significant, and I doubt it would have a large impact on the flavor development beyond the result of the temperatures themselves. You may want to look into passive cooling methods to keep summer temperatures down if you're able to build in your basement -- Gold's book "How and Why to Build a Wine Cellar" is an excellent resource for information on passively cooled spaces.

Also -- build yourself a "bottle thermometer" by inserting a thermometer into a glass bottle full of (boiled) water and sealing the top. You may find that the variation in the actual bottles is less than in the space itself.