my chassis has a rock in it due to the transmission tube bowing downwards which made my chassis lift a little and rock,if a peice of angle is placed in all four corner's the chassis is straight with a spirit level on my build table
i have an old friend who built sports cars for formula junior cars and clubmans car's and he said 5mm will be ok and even if you clamp your chassis tube's down soon as you unclamp it it can ping due to the heat