My question is, what effect does high temperatures have on the extraction of DNA from wheatgerm? Does lower temps increase the amount of DNA extracted?

Unfortunately, the answer to your question is that it depends on the extraction method. With an extraction method like http://learn.genetics.utah.edu/archive/wheatgerm/index.html
I suspect the detergent you used would be biggest variable.
Higher temperatures in the extraction method would work to help free the DNA from the other constituents, I don't think you should worry about DNA stability until the temperature is close to boiling & then the worry is denaturation. I don't think that low temperatures would be better other than to protect the DNA from DNase, but I don't think that is a significant concern.