It’s important to make sure your car is exactly 5 ounces on race day. But I’d suggest keeping it slightly underweight until you get to the race. It’s always easier to add weight than it is to remove it.
Last year our Scout District race was held outside on a car lot in April. Almost every car that showed up was weighing in at 5.1 ounces — a tenth of an ounce too heavy. One disgruntled father set out to prove that our scale was wrong and produced a 5 ounce brass tare weight. We put the weight on the scale and it came in at exactly 5 ounces. The scale was right, all the cars were too heavy.
Every one of these cars had raced previously in their Pack races. Were the scales at all the other races wrong? Had everyone added weight between races? Why were they coming in so heavy?