Temperature effects three aspects of your fermentation:
Flavor/Aromatic Profile, maceration (in reds) and yeast viability.
Vigorous yeast, which are most often supplied in kits, tend to ferment
fast and generate a lot of BTU's so trying to control the temperature
somewhat is good for many reasons. For whites, cooler temperature
will allow the development of esters from the yeast that with enchance
the aroma of the wine. It will also prevent off flavors and aromas,
by not stressing the yeast and therefore keeping the correct metabolic
pathways open. In reds the effect of warming temp[erature with
enchance thermal maceration, giving good procurement of grape tannins
and neccesary for good round mouthfeel. However, for many yeast
temperature above 85 can be lethal especially once alcohol levels
begin climbing. So yeast however can trive up to 100F.

The moral of the story is that constant temperature is the best rule
of thumb. Yes, whites are best fermented cooler than reds, but the
key is keeping the temp within a tight range to promote good
fermentations.