Realistic orbits are only limited by what the guy who implements orbits decides to do.
Calculating crude approximational per minute x,y positions from starting position, date and daysinyear variable I cannot imagine to be a too hard task, even using eliptical orbits. I'm far more worried about the flight time predictions, aka how many days it will take to get to Mars from Earth, considering both are moving. I'm pretty sure computational power won't be an issue on modern PCs, but this isn't simple. That said, I see no reason why this would not be done accurately.
As sublight said tho, while orbital wobbles can be done I am doubtful that THAT LEVEL of accuracy adds anything to the gameplay at all. Orbits will likely be according to pre-established paths without neighbor interference, unless someone can forward a reason why that would be needed.