That is a complex discusion on temperature-wise. Yes, negtive effect of increasing temp is the reduction of difference between water and oil, which redcuing effectiveness of separation. HOwever compare it to the effect of reduction of visicosity, it is minor to the whole separatin process. You may see it in Stoke equation.
Temp should be controled not only because of lossing valuable light content in oil, but also the heat engery. Depend on API gravity, the heavier oil, the higher temp. For light oil, it may not need to be heated for separation.