This is just my opinion on this issue. It's has been disgust here a few times and opinions vary. So here is mine, most cars sit at 2 to 4 degrees of rake. So if you add that to your caster, which I like to be around 7 or 8 degrees, your cross member would wind up at 9 to 12 degrees. I like a lot of caster, the car tracks better going down the road and it returns quicker after making a turn. The negative side of this is it is a little harder to turn the wheel at real slow speeds. You want correct caster without putting anything in a bind in your front end.
Ken