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 $220.81   11.646%   $246.5255   $246.53   $246.55   $246.50  
  Purchase #2 $138.75   13.357%   $157.2828   $157.28   $157.30   $157.30  
  Purchase #3 $58.57   4.733%   $61.3421   $61.34   $61.35   $61.30  
  Purchase #4 $95.61   11.321%   $106.434   $106.43   $106.45   $106.40  
  Purchase #5 $41.49   7.400%   $44.5603   $44.56   $44.55   $44.60  
  Purchase #6 $125.30   10.576%   $138.5517   $138.55   $138.55   $138.60  
  Purchase #7 $173.85   4.589%   $181.828   $181.83   $181.85   $181.80  
  7 purchase totals = $854.38       $936.5244   $936.52   $936.60   $936.50