Realism is a good goal, but don't lose sight of the trade-offs. If you wait to test until you have a perfect simulation, you'll miss opportunities to fix problems. Look for ways to start with simple testing instead, and then build up to more complex simulations.
Keep in mind that testing early is not a panacea, either. Problems can creep in late in development, and continuous testing can avoid last-minute fixes.
Read this TechBeacon article written by Load Impacts founder Ragnar Lönn, for more insight.