I'm assuming this is an electrical desalter/dehydrator you are talking about.
You want the temperature as high as possible as that reduces the oil viscosity (which promotes separation) and the density difference between the oil and water. 300F is typically the upper operating limit because of the electrical connections to the vessel. My information is many years old so you may want to talk to someone like Natco for more up to date advise.
Once you set your maximum temperature the pressure follows because you don't want any vapor breakout in the vessel so it depends somewhat on your crude.
Few other reasons for keeping temperature in lower range follow
1)to support water being kept in liquid phase ensuring maximum salts in solution phase;instead of some dropping(crystalisingor precipitating out)
2)With optimum pressures as otherwise all heat exchange equipments and even the de-salter vessel will require much costlier/heavier designs.
Thus to strike a balance with feasible/optimum initial capital cost along with better operating efficiency; moderately higher than water Normal boiling point temperatures are adopted ranging 130~140 Celsius(suitable in point pressure conditions).
There are companies that make a living supplying crude oil desalters. Google the subject for a listing....any one of them should be able to provide all the operational data you need.