Quote from jens2005:
I don't know which software package emulates limit order fills most realistically. Personally I use the paper trading account offered by Interactive Brokers and I noticed that prices generally have to trade through my limit before I'm getting filled. However, I don't know the exact algorithm so I can only speak based on my observations. Also my trading strategy isn't that sensitive to fills. If I exit a position I'm almost always pay the spread and if I enter a position I'm willing to change my limit price so that the market trades through it to make sure I got my position.