Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $132.45   4.707%   $138.6844   $138.68   $138.70   $138.70  
  Purchase #2 $6.44   5.746%   $6.81   $6.81   $6.80   $6.80  
  Purchase #3 $30.26   13.418%   $34.3203   $34.32   $34.30   $34.30  
  Purchase #4 $125.39   10.150%   $138.1171   $138.12   $138.10   $138.10  
  Purchase #5 $133.74   7.362%   $143.5859   $143.59   $143.60   $143.60  
  Purchase #6 $13.42   3.771%   $13.9261   $13.93   $13.95   $13.90  
  Purchase #7 $60.53   12.449%   $68.0654   $68.07   $68.05   $68.10  
  7 purchase totals = $502.23       $543.5092   $543.52   $543.50   $543.50