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 $148.89   12.786%   $167.9271   $167.93   $167.95   $167.90  
  Purchase #2 $146.84   9.231%   $160.3948   $160.39   $160.40   $160.40  
  Purchase #3 $117.52   12.386%   $132.076   $132.08   $132.10   $132.10  
  Purchase #4 $27.36   4.236%   $28.519   $28.52   $28.50   $28.50  
  Purchase #5 $227.95   9.489%   $249.5802   $249.58   $249.60   $249.60  
  Purchase #6 $78.53   9.843%   $86.2597   $86.26   $86.25   $86.30  
  Purchase #7 $53.76   4.668%   $56.2695   $56.27   $56.25   $56.30  
  7 purchase totals = $800.85       $881.0263   $881.03   $881.05   $881.10