It depends to a fair amount on what type of suspension you are running.
Most road cars are given + or - tolerances, but because of the built in compliance
actually run at just about parallel( back and front)when driving in "normal" road use otherwise the tyres would wear unevenly.
However as most race cars use either solid bushes or rose joints, the movement is more controlled . Whether your particular car needs tow in or out can only realy be ascertained by testing for the best all round handling and times.
In a race several years ago I had a slight "altercation" with another car, and found that it was steering better and put in faster lap times for the rest of the race and won the class. On checking the geometry in the workshop I found it a fair bit different than what I had been using in the past, and continued using that setting.
